Post-formable multilayer optical films and methods of forming

ABSTRACT

Articles including post-formed multilayer optical films with layers of at least one strain-induced birefringent material, methods of manufacturing such articles by post-forming multilayer optical films, and multilayer optical films that are particularly well-suited to post-forming operations are disclosed. The articles, methods and multilayer optical films of the present invention allow for post-forming of multilayer optical films including strain-induced index of refraction differentials while retaining the desired optical properties of the multilayer optical films.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/391,738, filed on Feb. 24, 2009, published as U.S. Patent ApplicationPublication No. 2009/0155540, now abandoned, which is a continuation ofU.S. patent application Ser. No. 10/883,059, filed Jun. 30, 2004, nowabandoned, which is a continuation of U.S. patent application Ser. No.10/115,559, filed on filed Apr. 3, 2002, now issued as U.S. Pat. No.6,788,463, which is a continuation of U.S. patent application Ser. No.09/126,917, filed Jul. 31, 1998, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 09/006,591,filed on Jan. 13, 1998, now issued as U.S. Pat. No. 6,531,230, and acontinuation-in-part of U.S. patent application Ser. No. 09/006,086,filed on Jan. 13, 1998, now issued as U.S. Pat. No 6,045,894, andincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of birefringent multilayeroptical films. More particularly, the present invention relates topost-formable multilayer optical films including at least onebirefringent material and methods of manufacturing post-formed articlesfrom multilayer optical films.

BACKGROUND OF THE INVENTION

Conventional methods of providing reflective objects typically includethe use of metal or substrates coated with thin layers of metals.Forming the articles completely of metal is typically expensive and mayalso suffer from other disadvantages such as increased weight, etc.Metal coated articles are typically plastic substrates coated with areflective metallic layer by vacuum, vapor or chemical deposition. Thesecoatings suffer from a number of problems including chipping or flakingof the metallic coating, as well as corrosion of the metallic layer.

One approach to addressing the need for reflective objects has been theuse of multilayer articles of polymers such as those discussed in U.S.Pat. No. 5,103,337(Schrenk et al.); U.S. Pat. No. 5,217,794 (Schrenk);U.S. Pat. No. 5,684,633 (Lutz et al.). These patents describe articles,typically films or sheets, that include multiple layers of polymershaving different indices of refraction and, as a result, reflect lightincident on the films. Although most of the listed patents recite thatthe articles are post-formable, only a few of them actually address themodifications needed to ensure that the articles retain their opticalproperties after forming. Among those modifications are the use ofdiscontinuous layers (U.S. Pat. No. 5,217,794) and increasing the numberof layers in the article or film (U.S. Pat. No. 5,448,404). Multilayerarticles including layers of birefringent materials, their opticalproperties and methods of manufacturing them are disclosed in, e.g., PCTPublication Nos. WO 97/01774 and WO 95/17303. This class of articlesincludes alternating layers of a birefringent material and a differentmaterial in which the refractive index differential between thealternating layers is caused, at least in part, by drawing of thearticle, typically provided in the form of a film. That drawing causesthe refractive index of the birefringent material to change, therebycausing the inter-layer refractive index differential to change. Thosestrain-induced refractive index differentials provide a number ofdesirable optical properties including the ability to reflect lightincident on the films from a wide range of angles, high reflectivityover broad ranges of wavelengths, the ability to control the reflectedand transmitted wavelengths, etc. For simplicity, multilayer articlesincluding one or more layers of birefringent materials will be referredto below as “multilayer optical films.”

None of the known multilayer articles and multilayer optical films andthe patents/publications describing them, however, address the problemsassociated with post-forming multilayer optical films. As discussedabove, multilayer optical films including alternating layers ofmaterials including at least one birefringent material rely onstrain-induced refractive index differentials.

Because multilayer optical films rely on refractive index differentialsdeveloped by drawing, post-forming of multilayer optical films poses anumber of problems. The additional strain caused during the post-formingprocesses can affect the refractive index differentials in themultilayer optical films, thereby affecting the optical properties ofthe multilayer optical films. For example, a multilayer optical filmdesigned to reflect light of one polarization orientation and transmitlight of the orthogonal polarization orientation may be altered duringpost-forming such that it reflects light with both polarizationorientations. In addition, many post-forming processes involve the useof heat during forming, and that heat may alter the strain-inducedcrystallization that serves as the basis for the refractive indexdifferentials in many multilayer optical films. As a result, themultilayer optical film may exhibit altered optical characteristics dueto the changed refractive index differentials. Furthermore, somemultilayer optical films including strain-induced birefringent layersmay be stretched to levels at or near their rupture or breaking pointsduring manufacturing. As a result, any further processing thatintroduces additional strain may well result in rupture of themultilayer optical films.

SUMMARY OF THE INVENTION

The present invention provides articles including post-formed multilayeroptical films including layers of at least one strain-inducedbirefringent material, methods of manufacturing such articles bypost-forming multilayer optical films, and multilayer optical films thatare particularly well-suited to post-forming operations. The articles,methods and multilayer optical films of the present invention allow forpost-forming of multilayer optical films including strain-induced indexof refraction differentials while retaining the desired opticalproperties of the multilayer optical films.

In one aspect, the present invention provides an article includingmultilayer optical film having an optical stack including a plurality oflayers, the layers comprising at least one birefringent polymer and atleast one different polymer, wherein the optical stack includes astrain-induced index of refraction differential along at least a firstin-plane axis, and further wherein the thickness of the optical stackvaries non-uniformly over the optical stack.

In another aspect, the present invention provides an article includingmultilayer optical film having an optical stack including a plurality oflayers, the layers including at least one birefringent polymer and atleast one different polymer, wherein the optical stack includes astrain-induced index of refraction differential along a first in-planeaxis and substantially the entire optical stack reflects at least about85% of light of desired wavelengths that is polarized along the firstin-plane axis, and further wherein the thickness of the optical stackvaries by at least about 10% or more.

In another aspect, the present invention provides an article includingmultilayer optical film having an optical stack including a plurality oflayers, the layers including at least one birefringent polymer and atleast one different polymer, wherein the optical stack includes astrain-induced index of refraction differential along a first in-planeaxis, and further wherein the optical stack defines first and secondmajor surfaces, the first major surface including at least one depressedarea formed therein.

In another aspect, the present invention provides an article includingmultilayer optical film having an optical stack including a plurality oflayers, the layers including at least one birefringent polymer and atleast one different polymer, wherein the optical stack includes astrain-induced index of refraction differential along a first in-planeaxis, wherein the thickness of the optical stack varies; and a substrateattached to the multilayer optical film.

In another aspect, the present invention provides a method ofmanufacturing an article including a multilayer optical film byproviding a multilayer optical film having an optical stack including aplurality of layers, the layers including at least one birefringentpolymer and at least one different polymer, wherein the optical stackexhibits a strain-induced index of refraction differential along a firstin-plane axis, and further wherein the optical stack has a firstthickness; and permanently deforming the optical stack from the firstthickness to a second thickness, wherein the optical stack exhibits apost-formed strain-induced index of refraction differential along thefirst in-plane axis after deformation.

In another aspect, the present invention provides a multilayer opticalfilm having a sequence of alternating layers of a birefringent polymerand a different polymer, the birefringent polymer including PEN, whereinthe birefringent polymer exhibits a total polarizability difference in arange of from at least about 0.002 up to about 0.018, and furtherwherein the birefringent polymer exhibits a maximum in-planebirefringence of about 0.17 or less.

In another aspect, the present invention provides a multilayer opticalfilm having a sequence of alternating layers of a birefringent polymerand a different polymer, the birefringent polymer including PET, whereinthe birefringent polymer exhibits a total polarizability difference in arange of from at least about 0.002 up to about 0.030, and furtherwherein the birefringent polymer exhibits a maximum in-planebirefringence of about 0.11 or less.

In another aspect, the present invention provides a method ofmanufacturing an article including a multilayer optical film byproviding a multilayer optical film with an optical stack that includesa plurality of layers, the layers including at least one birefringentpolymer and at least one different polymer, wherein the optical stackincludes a strain-induced index of refraction differential along atleast a first in-plane axis; and corrugating the optical stack to causea change in its visual appearance.

In another aspect the present invention provides an article including amultilayer optical film having an optical stack that includes aplurality of layers, the layers including at least one birefringentpolymer and at least one different polymer, wherein the optical stackincludes a strain-induced index of refraction differential along atleast a first in-plane axis, and further wherein the optical stack has acorrugated configuration.

These and other features and advantages of the present invention arediscussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one multilayer optical film accordingto the present invention.

FIG. 2 is a plan view of a portion of one post-formed multilayer opticalfilm according to the present invention including areas deformed alongtwo in-plane directions.

FIG. 2A is an enlarged partial cross-sectional view of the post-formedmultilayer optical film of FIG. 2 taken along line 2A-2A.

FIGS. 2B and 2C are enlarged partial cross-sectional views ofalternative post-formed multilayer optical films deformed along twoin-plane directions.

FIG. 3 is a plan view of a portion of one post-formed multilayer opticalfilm according to the present invention including areas deformed alongone in-plane direction.

FIG. 3A is an enlarged partial cross-sectional view of the post-formedmultilayer optical film of FIG. 3 taken along line 3A-3A.

FIGS. 3B and 3C are enlarged partial cross-sectional views ofalternative post-formed multilayer optical films deformed along onein-plane direction.

FIG. 4 is a perspective view of a portion of one post-formed multilayeroptical film according to the present invention.

FIG. 5 is an enlarged partial cross-sectional view of the multilayeroptical film of FIG. 4 taken along line 5-5 in FIG. 4.

FIG. 6 is a partial cross-sectional view of another post-formedmultilayer optical film according to the present invention.

FIG. 7 is a partial cross-sectional view of a headlight assemblyincluding post-formed multilayer optical film according to the presentinvention.

FIG. 8 is an enlarged cross-sectional view of one portion of theheadlight assembly of FIG. 7 taken along line 8-8.

FIG. 9 is an enlarged cross-sectional view of one portion of theheadlight assembly of FIG. 7 taken along line 9-9.

FIG. 10 is a plan view of one light guide including post-formedmultilayer optical film according to the present invention.

FIG. 11 is an enlarged partial cross-sectional view of the light guideof FIG. 10 taken along line 11-11.

FIG. 12 is a graph illustrating the relationship between draw ratio(horizontal axis) and crystallinity (vertical axis) in the birefringentmaterials of a multilayer optical film.

FIG. 12A illustrates the index of refraction in the direction of drawing(vertical axis) as a function of the draw ratio (horizontal axis) forone uniaxially drawn PEN film in which the orthogonal in-plane axisdimension is held generally constant.

FIG. 13 is a graph illustrating temperature (horizontal axis) versuscrystallization rate (vertical axis) for an exemplary birefringentmaterial.

FIG. 14 is a perspective view of an article including post-formedmultilayer optical film with selected areas having different opticalproperties.

FIG. 15 is a cross-sectional view of a composite including an multilayeroptical film and a substrate.

FIG. 16 is a plan view of the composite of FIG. 15 illustrating that thesubstrate may be provided in selected areas.

FIGS. 17 and 18 present the measured transmissions of light polarized inthe MD and TD directions, respectively, as discussed in Example 2.

FIG. 19 compares the spectra of cases 2, 5 and 6 as discussed in Example6.

FIG. 20 presents the block fractional transmissions for the three casesdiscussed in Example 7.

FIG. 21 is a partial schematic diagram of a corrugating apparatus usedin connection with Example 12.

FIG. 22 is a perspective view of the corrugated multilayer optical filmdiscussed in Example 12.

FIG. 23 is a perspective view of the corrugated multilayer optical filmdiscussed in Example 12 with undulations configured differently fromthose shown in FIG. 22.

FIG. 24 shows a plan view of a portion of a multilayer optical filmafter it has undergone a corrugation process such as discussed inExample 12.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

The present invention is directed at articles including post-formedmultilayer optical films including layers of at least one strain-inducedbirefringent material, methods of manufacturing such articles bypost-forming multilayer optical films, and multilayer optical films thatare particularly well-suited to post-forming operations. Post-forming ofmultilayer optical films presents problems because most, if not all,post-forming processes result in deformation of the film from itsmanufactured state. Those deformations can adversely affect the opticaland mechanical properties of the multilayer optical film.

While the present invention is frequently described herein withreference to the visible region of the spectrum, various embodiments ofthe present invention can be used to operate at different wavelengths(and thus frequencies) of electromagnetic radiation. For simplicity, theterm “light” will be used herein to refer to any electromagneticradiation (regardless of the wavelength/frequency of the electromagneticradiation) capable of being reflected by the multilayer optical films ofthe present invention. For example, the multilayer optical films may becapable of reflecting very high, ultrahigh, microwave and millimeterwave frequencies of electromagnetic radiation. More preferably, the term“light” will refer to electromagnetic radiation including theultraviolet through the infrared spectrum (including the visiblespectrum). Even more preferably, “light” as used in connection with thepresent invention can be defined as electromagnetic radiation in thevisible spectrum.

Furthermore, the multilayer optical films and processes of post-formingmultilayer optical films according to the present invention rely onstrain-induced index of refraction differentials between layers in thefilms. Typically, those differentials will not be expressed hereinnumerically. Where they are discussed with reference to specific indicesof refraction, however, it should be understood that the values used aredetermined using light having a wavelength of 632.8 nanometers.

As used herein, the terms “reflection” and “reflectance” and variationsthereof refer to the reflectance of light rays from a surface.Similarly, the terms “transmission” and “transmittance” and variationsthereof are used herein in reference to the transmission of lightthrough a surface, optical stack, film, etc. Except where dyes orcolorants are intentionally added, the optical stacks of the presentinvention preferably exhibit low or minimal absorption losses (typicallyless than 1% of incident light), and substantially all of the incidentlight that is not reflected from the surface of an optical stack will betransmitted therethrough.

As used herein, the term “extinction ratio” is defined to mean the ratioof total light transmitted in one polarization to the light transmittedin an orthogonal polarization.

Multilayer Optical Films

Many multilayer optical films used in connection with the presentinvention and methods of manufacturing them are described in U.S. Pat.No. 5,882,774 (Jonza et al.); U.S. Pat. No. 6,101,032 (Ouderkirk); U.S.Pat. No. 6,157,490 (Wheatley et al.); U.S. Pat. No. 6,207,260 (Wheatleyet al.); U.S. Ser. No. 09/006,288 (filed on Jan. 13, 1998, nowabandoned); U.S. Pat. No. 6,179,948 (Merrill et al.); and U.S. Ser. No.09/006,591 (filed on Jan. 13, 1998); as well as in various other patentsand patent applications referred to herein. Briefly, however, multilayeroptical films as used herein refers to optical films including at leastone birefringent material provided in contiguous layers with at leastone other material such that desired strain-induced refractive indexdifferentials are provided between the layers making up the films. Themultilayer optical films preferably exhibit relatively low absorption ofincident light, as well as high reflectivity for both off-axis andnormal light rays.

The reflective properties generally hold whether the films are used forpure reflection or reflective polarization of light. The uniqueproperties and advantages of multilayer optical films provides anopportunity to design highly reflective post-formed articles thatexhibit low absorption losses. One multilayer optical film used in themethods and articles of the present invention is illustrated in FIG. 1and includes a multilayer stack 10 having alternating layers of at leasttwo materials 12 and 14.

The multilayer optical films according to the present invention allinclude an optically active portion that will be referred to herein asthe “optical stack,” i.e., those layers that provide the desiredreflective properties of the multilayer optical films by virtue of therefractive index differentials within the optical stack. Other layersand/or materials may be provided in addition to the optical stack. Forexample, skin layers may be provided on the outside of the optical stackto improve the mechanical properties of the films or provide some otherdesired property or properties including secondary optical effects suchas retardation or polarization conversion, but the bulk of thereflective optical characteristics of the films are determined by theproperties of the optical stacks.

Although only two layers 12 and 14 are illustrated, it will beunderstood that the optical stack of the multilayer optical film 10 caninclude tens, hundreds or thousands of layers, and each layer can bemade from any of a number of different materials, provided that at leastone of the materials is birefringent. The characteristics whichdetermine the choice of materials for a particular optical stack dependupon the desired optical performance of the film. The optical stack maycontain as many materials as there are layers in the stack. For ease ofmanufacture, however, preferred optical thin film stacks contain only afew different materials. Some considerations relating to the selectionof materials for the optical stacks of multilayer optical films of thepresent invention are discussed below in the section entitled “MaterialsSelection.”

The boundaries between the materials, or chemically identical materialswith different physical properties, within the stack can be abrupt orgradual. Except for some simple cases with analytical solutions,analysis of the latter type of stratified media with continuouslyvarying index is usually treated as a much larger number of thinneruniform layers having abrupt boundaries but with only a small change inproperties between adjacent layers.

Further considerations relating to the selection of materials andmanufacturing of optical films can be obtained with reference to U.S.Pat. No. 5,882,774 (Jonza et al.); U.S. Pat. No. 6,157,490 (Wheatley etal.); U.S. Pat. No. 6,207,260 (Wheatley et al.); U.S. Ser. No.09/006,288 (filed on Jan. 13, 1998, now abandoned); U.S. Pat. No.6,179,948 (Merrill et al.); and U.S. Ser. No. 09/006,591 (filed on Jan.13, 1998).

The preferred optical stack is comprised of low/high index pairs of filmlayers, wherein each low/high index pair of layers has a combinedoptical thickness of ½ the center wavelength of the band it is designedto reflect at normal incidence. The optical thickness is the physicallayer thickness multiplied by the index of refraction of the material inthe layer for a given wavelength and polarization plane cross-section.Stacks of such films are commonly referred to as quarterwave stacks.

As indicated above, at least one of the materials is birefringent, suchthat the index of refraction (n) of the material along one direction isaffected by stretching the material along that direction. The indices ofrefraction for each layer are n1 x, n1 y, and n1 z for layer 12, and n2x, n2 y, and n2 z for layer 14. For the purposes of the presentinvention, the x and y axes will generally be considered to lie withinthe plane of the film and be perpendicular to each other. The z axiswill be perpendicular to both the x and y axes and will generally benormal to the plane of the film.

The stack 10 can be stretched in two (typically) perpendicular in-planedirections to biaxially orient the birefringent material in the layer14, or the stack 10 may be stretched in only one in-plane direction(uniaxially oriented). By stretching the multilayer stack over a rangeof uniaxial to biaxial orientation, a film can be created with a rangeof reflectivities for differently oriented incident light. Themultilayer stack can thus be made useful as reflective polarizers ormirrors.

If the stack 10 is stretched in the x and y directions, each adjacentpair of layers 12 and 14 exhibit refractive index differentials betweenlayers in each of the two mutually perpendicular in-plane directions (x& y). The values of the refractive index differentials can berepresented by Δx (which is equal to (n1 x−n2 x) where n1 x is greaterthan n2 x) and Δy (where Δy=n1 y−n2 y). It will be understood that areflective polarizer will preferably exhibit a Δx in stack 10 that issufficiently high to achieve the desired reflectivity and, further, thatthe stack 10 will exhibit a Δy that is sufficiently low such that asubstantial percentage of light with coincident polarization istransmitted.

An important parameter for improving the reflectivity of multilayeroptical films at oblique angles of incidence is the control of n1 z andn2 z in relation to the other indices. First assume that n1 x is thelarger of n1 x and n2 x such that Δx is positive and |Δx|>|Δy|. Toincrease the reflectivity of the multilayer optical stack at obliqueangles of incidence compared to normal incidence, it may be preferredthat Δz<Δx. More preferably, Δz≅0, and even more preferably Δz<0.

For reflective mirror films, the desired average transmission for lightof each polarization and plane of incidence generally depends upon theintended use of the reflective film. The average transmission at normalincidence for any polarization direction for a narrow bandwidthreflective film, e.g., a 100 nanometer bandwidth within the visiblespectrum is desirably less than 30%, preferably less than 20% and morepreferably less than 10%. A desirable average transmission along eachpolarization direction at normal incidence for a partial reflective filmranges anywhere from, for example, 10% to 50%, and can cover a bandwidthof anywhere between, for example, 100 nanometers and 450 nanometers,depending upon the particular application.

For a high efficiency reflective mirror film, average transmission atnormal incidence for any polarization direction over the visiblespectrum (400-700 nm) is desirably less than 10%, preferably less than5%, more preferably less than 2%, and even more preferably less than 1%.The average transmission at 60 degrees from the normal axis for anyplane of incidence and polarization direction for a high efficiencyreflective film from 400-700 nanometers is desirably less than 10%,preferably less than 5%, more preferably less than 2%, and even morepreferably less than 1%.

In addition, asymmetric reflective films may be desirable for certainapplications. In that case, average transmission for one polarizationdirection may be desirably less than, for example, 50%, while theaverage transmission along another polarization direction may bedesirably less than, for example 20%, over a bandwidth of, for example,the visible spectrum (400-700 nanometers), or over the visible spectrumand into the near infrared (e.g., 400-850 nanometers).

In summary, multilayer optical films used in the methods and articles ofthe present invention include a multilayer stack 10 having alternatinglayers of at least two diverse polymeric materials 12 and 14, at leastone of which preferably exhibits birefringence, such that the index ofrefraction of the birefringent material is affected by stretching. Theadjacent pairs of alternating layers preferably exhibit at least onestrain-induced refractive index differential (Δx, Δy) along at least oneof two perpendicular in-plane axes as discussed briefly below. Theselection of materials and/or the orientation process conditions can beused to control the value of Δz in relation to the values of Δx and Δy.

By stretching the multilayer stack over a range of uniaxial to biaxialorientation, a multilayer optical film can be created with a range ofreflectivities for differently oriented plane polarized light along withthe plane of incidence or polarization parallel to various film axes(typically corresponding to the stretch directions) based on the valuesof Δx, Δy, and Δz. Preferably, those refractive index differentials aregenerally uniform throughout the film to provide uniform opticalproperties throughout the film. Variations in those refractive indexdifferentials that fall below desired minimum values for the desiredoptical characteristics may cause undesirable variations in the opticalproperties of the films.

Although the articles including post-formed multilayer optical film, themethods of producing those articles, and the post-formable multilayeroptical films are often described or explained below with reference tomultilayer optical films designed to exhibit broadband reflectance overthe visible spectrum, it will be understood that the same concepts couldapply to articles, methods and films that exhibit reflectance of lighthaving any desired range or ranges of wavelengths and any desiredpolarizing qualities. In other words, the present invention is usefulwith both polarizing multilayer optical films (that preferentiallyreflect light of one polarization orientation while transmitting lightwith the orthogonal polarization orientation), as well as multilayeroptical films that provide uniform properties for light having anypolarization orientation.

Other optical films suitable for use in the post-forming process of thepresent invention include, for example, multilayer films and filmscomprised of a blend of immiscible materials having differing indices ofrefraction. Examples of suitable multilayer films include polarizers,visible and infrared mirrors, and color films such as those described inPatent Publications WO 95/17303, WO 96/19347, and WO 97/01440; filedapplications having U.S. Pat. No.; 6,531,230 (Weber et al.), U.S. Pat.No. 6,045,894 (Jonza et al.), U.S. Pat. No. 5,103,337(Schrenk), U.S.Pat. No. 5,122,905 (Wheatley et al), U.S. Pat. No. 5,122,906 (Wheatley),U.S. Pat. No. 5,126,880 (Wheatley), U.S. Pat. No. 5,217,794 (Schrenk),U.S. Pat. No. 5,233,465 (Schrenk), U.S. Pat. No. 5,262,894 (Wheatley),U.S. Pat. No. 5,278,694 (Wheatley), U.S. Pat. No. 5,339,198 (Wheatley),U.S. Pat. No. 5,360,659 (Arends), U.S. Pat. No. 5,448,404 (Schrenk),U.S. Pat. No. 5,486,949 (Schrenk) U.S. Pat. No. 4,162,343 (Wilcox), U.S.Pat. No. 5,089,318 (Shetty), U.S. Pat. No. 5,154,765 (Armanini), U.S.Pat. No. 3,711,176 (Alfrey, Jr. et al.); and Reissued U.S. Pat. No. RE31,780 (Cooper) and U.S. Pat. No. RE 34,605 (Schrenk), the contents ofwhich are incorporated herein by reference. Examples of optical filmscomprising immiscible blends of two or more polymeric materials includeblend constructions wherein the reflective and transmissive propertiesare obtained from the presence of discontinuous polymeric regions, suchas the blend mirrors and polarizers as described in Patent PublicationWO 97/32224, the contents of which is incorporated herein by reference.Preferred films are multilayer films having alternating layers of abirefringent material and a different material such that there is arefractive differential between the alternating layers. Especiallypreferred are multilayer films wherein the birefringent material iscapable of stress-induced birefringence, wherein the refractive indexdifferential between the alternating layers is caused, at least in part,by drawing the film. The drawing or similar forming process causes therefractive index of the birefringent material to change, thereby causingthe inter-layer refractive index differential to change. Thosestrain-induced refractive index differentials provide a number ofdesirable optical properties, including the ability to reflect lightincident on the films from a wide range of angles, high reflectivityover broad ranges of wavelengths, the ability to control the reflectedand transmitted wavelengths, etc.

Post-Forming of Optical Films

As used in connection with the present invention, post-forming caninclude a variety of processes designed to produce articles having avariety of shapes different from the smooth, planar-surfaced film shapeof the multilayer optical film as manufactured. Preferred manufacturingprocesses involve casting or otherwise forming the film, followed bystretching the film in one direction for a uniaxially stretched film. Ifthe film is to be biaxially stretched, it is typically stretched in boththe longitudinal (i.e., machine) direction and in the cross-webdirection although any two directions may be used (preferably twogenerally perpendicular directions). Both uniaxially and biaxiallystretched multilayer optical films are manufactured as generally smooth,planar films with caliper or thickness variations of about ±5% or lessas manufactured.

Post-forming, as discussed with respect to the present invention,involves further processing of the optical stacks in the multilayeroptical films to obtain some permanent deformation in the optical stack.The deformation will preferably involve thinning of the optical stackand it may also involve deforming at least one surface of the film fromthe uniformly smooth, planar-surfaced film shape in which it ismanufactured.

Because the deformations may cause the planarity of the optical stack tobe disrupted, it should be understood that, where discussed, thein-plane directions are considered to be relative to a localized area ofthe optical stack or a point on the optical stack. For a curved opticalstack, the in-plane axes can be considered to lie in a plane defined bythe tangent lines formed at a particular point on the optical stack. Thez-axis would then be perpendicular to that plane.

Post-forming may also include embossing in which the optical layers ofthe multilayer optical film, i.e., those layers responsible for thereflective properties of the multilayer optical film, are deformed toproduce a change in the optical properties of the film. Embossing thatprovides a textured surface to a skin layer without significantlyaffecting the optical properties of the optical layers within themultilayer optical film is not considered post-forming within themeaning of that term as used herein. Embossing of a multilayer coloredmirror films has been discussed in, e.g., U.S. patent application Ser.No. 08/999,624 (now abandoned) and U.S. Pat. No. 6,045,894 (Jonza etal.).

As can be seen in the embodiments discussed below, post-formed articlesare produced by deforming a generally smooth, planar-surfaced film orsheet material to an article having three-dimensional characteristics.Articles including post-formed multilayer optical film can includepost-formed multilayer optical film having relatively small deformationssuch as those experienced as a result of embossing the optical layers ofthe multilayer optical film, up to larger scale deformations such asthermoformed multilayer optical film used in, e.g., a deep lamp cavity,having a high aspect ratio (i.e., depth to width ratio).

Post-forming operations will typically, but not necessarily, employ heatto improve the working qualities of the multilayer optical film. Thepost-forming processes may also employ pressure, vacuum, molds, etc. tofurther improve the working qualities of the multilayer optical film, aswell as increase the throughput of the process. For example, one typicalpost-forming method is thermoforming, including the various forms ofvacuum or pressure molding/forming, plug molding, etc. Post-forming mayalso include re-drawing or stretching films or portions/areas of filmsin planar directions or stretching the films into non-planar or curvedshapes.

It may be helpful to further describe post-forming in terms of theamount of draw induced in the optical stack. In general, post-formingcan involve a texturing of the optical stack, shallow drawing of theoptical stack, and deep drawing of the optical stack. In the cases wherethe post-forming involves texturing and/or shallow drawing, it may bepossible to use both fully drawn and underdrawn multilayer optical films(as described below) to perform the methods because the draw ratios tobe experienced may be relatively small. When performing deep draws,however, it may be advantageous to use underdrawn optical stacks becauseof their increased extensibility as compared to fully-drawn multilayeroptical films. Some exemplary post-forming processes and the articlesmanufactured thereby are presented below.

One approach to characterizing deformation of the optical stack in apost-formed multilayer optical film according to the present inventionis depicted in FIGS. 2 and 2A-2C. The optical stack 20 includes a firstmajor side 24 and a second major side 26 (see FIG. 2A). Also illustratedare selected areas 22 in which the optical stack 20 has been deformed.The selected areas 22 are depicted as being substantially uniform insize and arranged in regular, repeating pattern. It will however, beunderstood that the selected areas 22 may be non-uniform and/or providedin pattern that irregular/non-repeating.

One of the selected areas 22 and the surrounding optical stack 20 isseen in the enlarged, partial cross-sectional view of FIG. 2A. Theresult of the post-forming is that the thickness of the optical stack 20varies. One of the ways in which that variation can manifest itself isthat each of the selected areas 22 can form a depression in theotherwise generally smooth, planar first major side 24 of the opticalstack 20. This post-forming may be considered as one example oftexturing, i.e., causing deformations in one surface 24 of the opticalstack 20 that do not necessarily find any corresponding deformation onthe opposite surface 26 of the optical stack 20. Texturing does,however, differ from embossing of skin layers in that the optical stack20 is itself deformed.

Another manifestation of the thickness variations in an optical stack120 is illustrated in FIG. 2B where both the first and second majorsides 124 and 126 are deformed in selected areas 122 and 128. Likeselected area 122 on the first major side 124, selected area 128 on thesecond major side 126 is also formed as a depression in the otherwisegenerally smooth planar second major side 126. This is one example of ashallow draw that could be caused by pressure or by strain.

Yet another manifestation of the thickness variations in an opticalstack 220 is illustrated in FIG. 2C where both the first and secondmajor sides 224 and 226 are deformed in selected areas 222 and 228.While selected areas 222 are formed as depressions on the first majorside 224, the selected area 227 on the second major side 226 is formedas a raised area extending outwards from the otherwise generally smooth,planar second major side 226. As depicted, it may be preferred that theraised area 228 on the second major side 226 be located opposite thedepressed area 222 on the first major side 224.

The post-forming result depicted in FIG. 2C is another example of whatcould be considered a shallow draw, i.e., deformation of the opticalstack 220 in the opposing sides 224 and 226 of the optical body 220.

FIG. 3 and cross-sectional views 3A-3C illustrate an alternativeembodiment of a post-formed multilayer optical film according to thepresent invention. The optical stack 20′ includes a first major side 24′and a second major side 26′ (see FIG. 3A). Also illustrated are selectedareas 22′ in which the optical stack 20′ has been deformed. The selectedareas 22′ are depicted as being substantially uniform in size. It willhowever, be understood that the selected areas 22′ may be non-uniform.

Referring back to FIG. 2, the selected areas 22 of optical stack 20 aredeformed along both in-plane axes (x & y). In contrast, the selectedareas 22′ of optical stack 20′ are preferably deformed along only onein-plane axis (the x axis in FIG. 3). If the optical stack 20′ isdesigned to operate as a reflective polarizer in the deformed areas 22′,it may be desirable to deform those areas in the direction of maximumindex difference. That should reduce post-forming extension in thematched refractive index direction. As a result, the reflectiveperformance of the polarizing optical stack 20′ may be better maintainedand, in some cases, increased extension along the proper direction mayincrease the desired reflectivity of the optical stack 20′.

One of the selected areas 22′ and the surrounding optical stack 20′ isseen in the enlarged, partial cross-sectional view of FIG. 3A. Theresult of the post-forming is that the thickness of the optical stack20′ varies. One of the ways in which that variation can manifest itselfis that each of the selected areas 22′ can form a depression in theotherwise generally smooth, planar first major side 24′ of the opticalstack 20′.

Another manifestation of the thickness variations in an optical stack120′ is illustrated in FIG. 3B where both the first and second majorsides 124′ and 126′ are deformed in selected areas 122′ and 128′. Likeselected area 122′ on the first major side 124′, selected area 128′ onthe second major side 126′ is also formed as a depression in theotherwise generally smooth, planar second major side 126′.

Yet another manifestation of the thickness variations in an opticalstack 220′ is illustrated in FIG. 3C where both the first and secondmajor sides 224′ and 226′ are deformed in selected areas 222′ and 228′.While selected areas 222′ are formed as depressions on the first majorside 224′, the selected area 227′ on the second major side 226′ isformed as a raised area extending outwards from the otherwise generallysmooth, planar second major side 226′. As depicted, it may be preferredthat the raised area 227′ on the second major side 226′ be locatedopposite the depressed area 222′ on the first major side 224′.

The deformations illustrated in FIGS. 2A-2C and 3A-3C can becharacterized by the ratio of the thickness t_(o) in the undeformedportions of the optical stacks to the thickness t_(f) of the deformedportions of the optical stacks. Both of those thicknesses are preferablymeasured between the major surfaces of the optical stacks, i.e., thethickness of any skin layers is not considered. Typically, it may bedesirable that the ratio t_(o):t_(f) be at least about 1.1:1 or greater.In some cases, it is desirable that the ratio t_(o):t_(f) be at leastabout 1.5:1 or greater, more preferably at least about 1.75:1 orgreater, and even more preferably at least about 2:1 or greater.

FIGS. 4 & 5 illustrate a more extreme example of the post-formed opticalstack 220 illustrated in FIG. 2C. The post-formed optical stack 30illustrated in FIGS. 4 & 5 can be considered an example of a deep drawpost-forming process. The optical stack 30 of FIG. 4 includes a firstmajor side 34 (see FIG. 5) and a second major side 36 along with aplurality of selected areas 32 in which the optical stack 30 has beenpost-formed to provide depressed areas 32 formed on the first major side34 of the optical stack and raised areas 37 formed on the second majorside 36 of the optical stack 30.

The deformed areas of the deeply drawn optical stack can becharacterized by the aspect ratio of the width (w) of the depressedareas 32 as measured across the opening 33 of the depressed area 32 tothe depth (d) of the depressed areas 32 as measured from the first majorside 34 of the optical stack 30. It is preferred that the width of thedepressed area 32 be measured across its narrowest dimension. It may bedesirable that the depressed areas 32 have an aspect ratio w:d of about10:1 or less, more desirably 2:1 or less, even more desirably about 1:1or less, and still more desirably about 0.5:1 or less.

Alternatively, the deformation in the optical stack 30 can be measuredin absolute terms. For example, it may be preferred that the depth d beat least about 0.1 millimeter or more; more preferably at least about 1millimeter or more; and even more preferably at least about 10millimeters or more. It will be understood that where the depth d of thedepressed areas 32 approaches or exceeds the thickness of the opticalstack 30, the more likely it is that a raised area 37 will be formed onthe second major side 36 of the optical stack.

The measurement of the depth d of the depressed areas 32 formed on thefirst major side 34 of the optical stack 30 is not limited to thoseinstances in which the first major side is planar. Turning now to FIG.6, where the optical stack 130 of a multilayer optical film is depictedin a curved configuration. The optical stack 130 includes a depressedarea 132 formed on the first major side 134 of the optical stack 130 anda corresponding raised area 137 on the second major side 136 of theoptical stack 130. The depth d of the depressed area 132 will preferablybe measured from the geometric surface defined by the first major side134 of the optical stack 130 and will typically be the largest depthfrom that geometric surface.

FIGS. 7-9 depict another illustrative article including post-formedmultilayer optical film. FIG. 7 is a cross-sectional view of a headlightassembly 40 for, e.g., an automobile or truck. The headlight assembly 40includes a lens 42, a lamp cavity 44 having a reflective inner surface46, and a light source 48 mounted within the lamp cavity 44.

It is preferred that the reflective inner surface 46 of the lamp cavity44 include post-formed multilayer optical mirror film manufacturedaccording to the principles of the present invention. In thisembodiment, it is preferred that the multilayer optical film used behighly reflective for visible light and it may also be helpful if themultilayer optical film is also reflective for light into the infraredspectrum to limit heat build-up of the lamp cavity 44 due to absorptionof infrared energy by the substrate on which the reflective innersurface 46 is located. Alternatively, if the multilayer optical film hassufficient structural integrity such that entire lamp cavity 44 isconstructed of the multilayer optical film, it may be preferable thatthe multilayer optical film be transmissive for infrared energy to limitheat build-up within the headlight assembly 40.

FIG. 8 is an enlarged cross-sectional view of the lamp cavity 44 takenalong line 8-8 in FIG. 7, and FIG. 9 is an enlarged cross-sectional viewof the lamp cavity 40 taken along line 9-9 in FIG. 7. Both of the viewsdepict a layer of post-formed multilayer optical film 50 on the innersurface 46 of the lamp cavity 44. Because the multilayer optical film 50typically lacks sufficient structural rigidity alone, it may bepreferred to mount the multilayer optical film 50 on a substrate 52 orsome other form of structural support, e.g. a frame, etc., by anysuitable technique. Alternatively, the multilayer optical film can belaminated to or coextruded with a thicker layer that provides structuralrigidity either before or after post-forming operations.

Post-forming processes do not typically deform a multilayer optical filmuniformly and, as a result, the thickness of the optical stacks inpost-formed multilayer optical films according to the present inventionvary. The variations in thickness of the post-formed multilayer opticalfilm are in direct contrast with the controlled uniform thickness of themultilayer optical film as manufactured. That uniform thickness isdesired because the thickness of the optical layers within themultilayer optical film define, in part, its optical properties. As aresult, variations in the multilayer optical film as manufactured arenot desired because they can adversely impact the uniform opticalproperties of the film. For example, non-uniformities in the opticalstack of multilayer optical film as manufactured can result iniridescence or other optical artifacts.

Thickness variations in the optical stack of post-formed multilayeroptical film are, in large part, caused by variations in the strainexperienced in different areas of the multilayer optical film duringpost-forming. In other words, some areas of the post-formed multilayeroptical film may experience significant deformation (strain) while otherareas may experience little or no deformation during post-forming.

The optical stacks of post-formed multilayer optical film in articleswill, as a result, often include variations in thickness as illustratedin FIGS. 3A-3C, 8 and 9. For example, the thickness of the multilayeroptical film 50 varies between the two points in the lamp cavity 44. Thethickness t₁ of the optical stack of the post-formed multilayer opticalfilm seen in FIG. 8 is thicker than the thickness t₂ of the opticalstack of the post-formed multilayer optical film depicted in FIG. 9. Inboth areas, however, it is preferred that the reflectivity of themultilayer optical film 50 for the desired range of wavelengths remainhigh for normal, as well as off-axis, light. The importance of off-axisreflectivity can be seen in FIG. 7 where light from the light source 48may approach portions of the light cavity 44 at high angles off ofnormal.

The thickness variations in the optical stack can cause what is commonlyreferred to as band shifting. In other words, the range of wavelengthsof which any multilayer optical film is reflective is, in part, afunction of the physical thickness of the layers in the multilayeroptical film. Varying the physical thickness of the layers can cause therange of wavelengths over which the film is reflective to change.Because changes in thickness typically involve thinning of themultilayer optical film from its manufactured thickness, band shiftingis usually downward. For example, a multilayer optical film thatexhibits broadband reflectance of light with wavelengths over the rangeof 400-900 nanometers and is thinned by a factor of 2 duringpost-forming will, after thinning, typically exhibit broadbandreflectance for light with wavelengths in the range of 200-450nanometers.

One approach to compensate for the effects of thinning multilayeroptical films (or any multilayer article exhibiting reflectivity as aresult of refractive index differentials, is discussed in U.S. Pat. No.5,448,404 (Schrenk et al.). Essentially, the thinning effect andcorresponding band shift can be compensated for by adjusting thebandwidth of the multilayer optical film as manufactured such that,after post-forming, the multilayer optical film has layers with theappropriate optical thickness to reflect light with the desiredwavelengths.

Although both the upper and lower band edges may be adjusted tocompensate for thinning, for broadband mirrors it may be preferable toadjust only the upper edge of the range of reflected wavelengths upwardby a factor that is at least as large as the expected maximum factor bywhich the multilayer optical film will be thinned during post-forming.By increasing the upper limit of the range of wavelengths over which themultilayer optical film reflects light before post-forming or drawing,the portions of the post-formed multilayer optical film that are thinnedduring post-forming will maintain their reflectivity over the desiredrange of wavelengths (assuming the maximum factor by which themultilayer optical film is thinned during post-forming does not exceedthe factor by which the upper limit of the wavelength range has beenadjusted to account for thinning during post-forming).

For broad band mirrors, it is typically not preferred to adjust thelower limit in the reflected wavelength range because some areas of themultilayer optical film may experience little or no deformation orthinning during post-forming. By supplying a multilayer optical filmthat, before post-forming, already reflects light at the lower end ofthe desired range of wavelengths, reflectivity of the entire post-formedmultilayer optical film at the lower end of the desired range ofwavelengths can be retained after post-forming.

For example, if the post-formed multilayer optical film in the articleis to reflect substantially all visible light (i.e., 400-700 nanometerlight), then before post-forming the multilayer optical film shouldreflect normal incident light in at least the wavelength range of fromabout 400 nanometers to about 900 nanometers multiplied by the expectedthinning factor (the increase in the upper edge bandwidth from 700 to900 nanometers is provided to compensate for light approaching at anglesoff of the normal axis). If the maximum factor by which the post-formedmultilayer optical film is expected to be thinned during post-forming is2, then the multilayer optical film will preferably reflect normalincident light in at least the wavelength range of from about 400nanometers to about 1800 nanometers. If the maximum factor by which thepost-formed multilayer optical film is expected to be thinned duringpost-forming is 3, then the multilayer optical film will preferablyreflect normal incident light in at least the wavelength range of fromabout 400 nanometers to about 2700 nanometers.

If the optical stack of a multilayer optical film is designed tocompensate for thinning, variations in the thickness of the post-formedmultilayer optical film can be allowed without significantly affectingreflectivity of the optical stack over the desired wavelengths. Forexample, the ratio t₁:t₂ in the post-formed multilayer optical filmarticle 50 illustrated in FIGS. 7-9 may be at least about 2:1 or morewithout significantly affecting the reflective properties of themultilayer optical film. In some cases, it may be possible to providemultilayer optical films that can support thickness ratios of 3:1 ormore without significant degradation of the optical properties of thepost-formed multilayer optical film over desired wavelengths.

FIGS. 10 & 11 illustrate another post-formed article according to thepresent invention. The article 70 is a light guide that can distributelight from a single source 72 to a plurality of distribution points 74a, 74 b and 74 c (collectively referred to as distribution points 74).Light guide 70 could be used in, e.g., lighting an instrument panel inan automobile or the like.

As seen best in the cross-sectional view of FIG. 11, the light guide 70can be formed from film 76 that has been post-formed into the desiredshape. Bonded over the post-formed film 76 is a cover film 78 that, inthe depicted embodiment, is a substantially planar sheet of film 78. Itwill, however, be understood that the cover film 78 could also bepost-formed if desired. Different areas of the post-formed film 76and/or the cover film 78 can be post-formed to varying thicknesses toallow for the transmission of light of different wavelengths (e.g.,visible light with different colors). The two multilayer optical films76 and 78 can be bonded using a variety of techniques. In the depictedembodiment, the films 76 and 78 are adhesively bonded using an adhesive77. Other techniques for bonding include mechanical fasteners or clamps,welding, etc.

Although some specific examples of articles including post-formedmultilayer optical film have been described above, it will be understoodthat post-formed multilayer optical film may be included in any articlein which it is desired to take advantage of the unique opticalproperties of multilayer optical films. For example, articles includingpost-formed multilayer optical film may find use in the automotive areafor headlights, taillights, and other areas where the reflectiveproperties of the post-formed articles according to the presentinvention would be advantageous. In addition, post-formed articles couldalso be used in the automotive industry as trim pieces for head lamps,bezels, knobs, automotive trim, and the like. The articles may also findapplication in trim articles such as the light work for consumerappliances including refrigerators, dishwashers, washers, dryers,radios, and the like. They may also find use as toys or novelty items.Other applications for post-formed articles according to the presentinvention include light guides and/or pipes, shaped reflectors forexterior lighting applications, bulb reflectors for use in, e.g.,backlit computer displays, medical/dental instruments other than thosedescribed herein (e.g., disposable laparoscopic mirrors), etc. In stillother applications, the post-formed articles may provide colored mirrorsor filters for use in, e.g., center high mount stop lamps, decals, hoodornaments, etc. Other uses include jewelry, seasonal ornaments (e.g.,Christmas tree ornaments), graphics, textured coatings, etc.

The post-formed articles of the present invention may also be used asdecorative items. Decorative items that may be formed from thecorrugated films include ribbons, bows, wrapping paper, gift bags,garlands, streamers, centerpieces, and ornaments. The post-formedarticles may also be employed in a gift box or other decorativepackaging (e.g., cosmetic or food packaging), yarns, or they may bearranged as a window in a gift bag. These examples of decorative itemsare presented for illustrative purposes only and should not be construedas a limitation on the variety of decorative items in which thepost-formed articles of the present invention may be employed.

Furthermore, the articles according to the present invention may beconstructed entirely of post-formed multilayer optical film or they mayonly include multilayer optical film in their construction. If thepost-formed multilayer optical film constitutes only a portion of thearticle, it will be understood that the post-formed multilayer opticalfilm could be integrated into larger assemblies by any suitabletechniques, such as insert injection molding, ultrasonic welding,adhesive bonding, and other techniques.

Underdrawn Multilayer Optical Films

Of the multilayer optical films described in U.S. Pat. No. 5,882,774(Jonza et al.), the mirror constructions of such films are typicallyoptimized for a high index differential. The films typically have lowextensibility limits (i.e., those limits beyond which the filmstypically deform without fracture or tear during deformation) becausethey are stretched during manufacturing to levels that provide thedesired high index of refraction differential. In addition, some of themultilayer optical films may be heat-set during manufacturing. Heatsetting induces further crystallization within the film and thatincreased crystallization will typically further reduce theextensibility limits of the films.

As a result of their relatively low extensibility limits, knownmultilayer optical films such as those described in U.S. Pat. No.5,882,774 (Jonza et al.) may be difficult to post-form without resultingin significant negative effects on the optical properties of theresulting post-formed multilayer optical film. Although the methodsdescribed above may be helpful in providing articles includingpost-formed multilayer optical film and methods of forming the articles,another approach to providing articles including post-formed multilayeroptical films can be pursued.

That other approach involves using multilayer optical films in which theextensibility limits of the film are increased for post-forming bydeliberate underdrawing of the film during its manufacture to producewhat will be described with respect to the present invention as an“underdrawn multilayer optical film” or “underdrawn film”. Suchunderdrawn multilayer optical film can then be provided in a rolls orsheets for use in a subsequent post-forming process or it may bedirected into an in-line post-forming process.

Multilayer optical film including layers of one or more birefringentmaterials alternating with another material may be characterizedaccording to the strain-induced orientation and/or crystallinity of thebirefringent materials in the films. In fully drawn films, or at leastfilms considered to be fully drawn for the purposes of the presentinvention, the birefringent materials will typically exhibit higherlevels of orientation and/or crystallinity than a correspondingmultilayer optical film constructed of the same materials that isunderdrawn.

The higher level of crystallinity in the fully drawn films is, in largepart, the result of the increased effective strain to which themultilayer optical film is subjected during manufacturing. As discussedabove, fully drawn films are typically drawn to higher levels to improvetheir reflective properties. Those reflective properties are largelybased on the orientation and/or crystallinity of the birefringentmaterials in the film, which can be correlated to the index ofrefraction of the birefringent materials. As a result, orientationand/or crystallinity are also related to the refractive indexdifferentials (Δx, Δy) in any multilayer optical film.

Because an underdrawn multilayer optical film is not subjected to thesame level of effective strain as is a fully drawn multilayer opticalfilm with the same construction, the birefringent material in theunderdrawn multilayer optical film will typically exhibit reducedcrystallinity or at least one reduced in-plane refractive indexdifferential (Δx or Δy) as compared to a fully drawn multilayer opticalfilm manufactured with the same materials, layer thicknesses, numbers oflayers, etc.

The reduced orientation and/or crystallinity may also typically resultin reduced refractive index differentials in the underdrawn multilayeroptical films as compared to the same construction in a fully drawnstate. As a result, it may be helpful to increase the number of layersusually required to cover a given wavelength range with a givenreflectance. Second order peaks from the thicker layers of the broaderband may reduce the actual need for an increase in the layer numbers.Such considerations can, however, be determined based on the discussionsin U.S. Pat. No. 5,882,774 (Jonza et al.).

It is important to note that, in addition to an upper limit oncrystallinity for an underdrawn multilayer optical film, there is alsopreferably a lower limit as well. In other words, an underdrawnmultilayer optical film including birefringent materials in its layerswill include at least some level of strain-induced crystallinity. Byproviding underdrawn multilayer optical films with at least some levelof strain-induced crystallinity, the post-forming of the underdrawnmultilayer optical films will typically be more predictable as comparedto a film in which no strain-induced crystallization is found in thebirefringent materials.

The importance of providing an underdrawn multilayer optical film withat least some strain-induced crystallinity is illustrated in FIG. 12, anidealized graph of draw ratio (horizontal axis) versus crystallinity(vertical axis) for multilayer optical films including layers of atleast one birefringent material alternating with another material. Thebehavior illustrated in FIG. 12 is typical of polyesters such as PEN,PET or co-polymers comprising them which can develop birefringence andwhich can be cast from a die and quenched efficiently resulting in aninitial cast web or film with very little crystallinity. FIG. 12 mayalso characterize other quenchable, birefringent polymeric materialsthat are susceptible to strain-induced crystallization. Again, suchquenched films would preferably exhibit only low levels of crystallinitycaused by crystallization during quenching prior to drawing. As drawingof the film is begun, the crystallinity of the birefringent materials inthe multilayer optical film may begin to increase, but the increases areat relatively low initial rates. Those draw ratios at which thestrain-induced crystallinity increases at a relatively low initial rateare included in what will be defined as Regime I for the purposes of thepresent invention. As the draw ratio increases past Regime I into whatwill be referred to as Regime II, the crystallinity of the birefringentmaterial in the multilayer optical film as a function of the draw ratioincreases at a significantly faster rate than in Regime I.

In Regime I of FIG. 12, the effect of drawing is approximatelyreversible in as much as cessation of drawing and continued heatingallows for the relaxation of orientation (i.e. a reduction in the indexof refraction differences in the three principal material directions)with minimal crystallization. The reversibility is not necessarilycomplete because Regime I typically appears in a temperature region oflarge supercooling. Thus crystallization is thermodynamically favoredbut kinetically hampered. Accumulated time during drawing and relaxationat these temperatures (e.g. via cycling) may eventually bring thematerial into Regime II via the relatively slow accumulation ofcrystallinity. Nevertheless, it is this approximate reversibility thatdistinguishes Regime I from Regime II. In general, the degree ofcrystallinity (or total polarizability as described later) tolerable inthis regime depends on the particular polymer, its quenching conditionsand its pre-drawing post process conditions.

The draw ratio at which the rate of crystallization of the birefringentmaterial in the multilayer optical film begins to increase significantlyand move into Regime II can be influenced by a number of factorsincluding draw rate, temperature, etc. After the birefringent materialhas experienced sufficient strain-induced crystallization to enterRegime II, however, it will typically follow the crystallization curvedefined by that initial drawing. In other words, the film cannotcontinue to be drawn without inducing crystallization in thebirefringent materials at the increased rates associated with Regime IIin the graph of FIG. 12. As a result, the characteristics of the filmwill be subject to less variability when drawn further in post-formingprocesses because the crystallization rate of the birefringent materialsis, in large part, set by the pre-stretching required to put the filminto Regime II.

For a multilayer optical film including birefringent materials that havenot experienced sufficient strain-induced crystallization to enterRegime II, further stretching or drawing during post-forming will not beas predictable because the point at which the crystallization ratestarts to increase significantly is subject to the factors listed above,e.g., temperature and draw rate. As a result, the film could experiencesmall increases in the draw ratio that result in significant increasesin the rate of crystallization of the birefringent materials or it couldexperience large draw ratios with a relatively small increase in therate of crystallization of the birefringent materials. In either case,the level of predictability is reduced as compared to a film thatincludes sufficient strain-induced crystallization such that its rate ofcrystallization is largely set, i.e., the birefringent materials in themultilayer optical film have entered Regime II.

In the case of many polymers, especially the polyesters including PEN,PET and copolymers including PEN and/or PET, a third regime develops inwhich the index of refraction increases at a much slower rate withrespect to the draw ratio. Often the total polarizability will alsochange at a much slower rate as well. FIG. 12A illustrates the index ofrefraction in the direction of drawing (vertical axis) as a function ofthe measured draw ratio (horizontal axis) for one uniaxially drawn PENfilm in which the orthogonal in-plane axis dimension is held generallyconstant. The PEN used for this illustrative case had an intrinsicviscosity of 0.48 and was drawn according to a linear draw profile of20% per second initial draw rate at 130 degrees Celsius.

For the illustrated case, Regime II begins at a draw ratio of about two(2) and Regime III begins at a draw ratio of about three (3). The onsetof these regimes depends on process and material conditions including,for example, raising the strain rate, raising the intrinsic viscosity,lowering the temperature, and/or lowering the glass transitiontemperature (e.g., by lowering the moisture and/or plasticizer content)may all lower the draw ratio at onset for Regimes II and III from thoseillustrated in FIG. 12A. The molecular weight distribution, rather thanjust an intrinsic viscosity may also alter the regime onsets. Analogousresults can be expected for biaxially drawn films.

In view of the above discussion, one difference between a fully drawnmultilayer optical film and an underdrawn multilayer optical film of thesame construction is that the fully drawn multilayer optical filmincludes birefringent materials in which the crystallinity is higherthan the crystallinity of the birefringent materials in the underdrawnmultilayer optical films. Where the birefringent material in themultilayer optical film is a polyester, it may be preferred that thecrystallinity of the birefringent polymer is about 18% or less, morepreferably about 15% or less. In comparison, the crystallinity of thesame birefringent polyesters in the fully drawn multilayer optical filmswill be at least about 20% or more, more typically about 25% or more.

In addition to an upper limit for crystallinity, underdrawn films canalso be characterized by a lower limit for the crystallinity of thebirefringent materials in the underdrawn multilayer optical film,because the birefringent materials in the films do preferably exhibitsome level of strain-induced crystallinity. In other words, it ispreferred that the birefringent materials in the multilayer opticalfilms have entered Regime II as discussed above. For multilayer opticalfilms including polyesters as the birefringent materials, it may bepreferred that the lower limit of crystallinity of the birefringentmaterials in the multilayer optical film be at least about 3% or more,in some instances more preferably at least about 5% or more, and inother instances even more preferably at least about 10% or more. Higherlevels of crystallinity typically provide higher levels of birefringencein the underdrawn state and reflect the degree of underdrawing. Higherbirefringence can improve the performance of the initial underdrawnstate in a finished post-formed article.

Although we do not wish to be limited by any particular theory, it isbelieved that the lowest level of crystallinity provides a minimum levelof connectivity between the micro-crystalline domains, e.g. via tiechains, which substantially reduces the propensity for large scalerelaxation of the developing morphology. In many instances,crystallization at these levels will move the birefringent materials inthe multilayer optical film into Regime II. The exact threshold of lowercrystallinity depends upon the chemical nature of the material includingthe composition and molecular weight as well as upon the processconditions such as temperature, rate and duration of draw and heating

Although crystallinity may be used to characterize underdrawn multilayeroptical films, underdrawn multilayer optical films may alternatively becharacterized using what will be referred to herein as “totalpolarizability” of the layers including birefringent materials.Determination of total polarizability is based on the refractive indicesof the layer or layers including birefringent materials within themultilayer optical film.

The “total polarizability difference” will be defined as the differencebetween the total polarizability of the drawn material and that of thequenched amorphous state of the same material. Any given material isexpected to possess a maximum total polarizability difference in acertain maximal fully drawn state. Where the multilayer optical filmincludes two or more different layers with different compositions ofbirefringent materials, total polarizability difference will preferablybe measured for the layers including birefringent materials with thelargest total polarizability difference relative to its maximum totalpolarizability difference as determined by the methods discussed below.

Refractive indices may be measured by a variety of standard methodsusing, e.g., an Abbe refractometer or a prism coupling device (e.g. asavailable from Metricon, Piscataway, N.J.). Although it is difficult todirectly measure the refractive indices of the materials in theindividual layers of the optical stack of the multilayer optical film,the refractive indices of the optical stack as a whole can be reliablymeasured. Furthermore, the refractive indices of the optical stack as awhole are weighted averages of the refractive indices of the materialsin each of the individual layers making up the optical stack.

If, for example, the optical stack is constructed of two or morematerials, the interdiffusional effects between layers are small, andthe refractive indices of only one of the materials changessignificantly in response to drawing, then the refractive indices of theindividual layers can be estimated based on the refractive indices ofthe optical stack as a whole. These estimates are based on the typicallyaccepted assumption that the refractive indices of the optical stack asa whole are the optical-thickness-weighted averages of the refractiveindices of the materials in the various layers of the optical stack.

In another variation, in those films in which one or more of thematerials making up the layers of the optical stack are also present inthicker skin layers and/or internal protective boundary layers, then itcan typically be assumed that the refractive indices are the same forthe same material, whether that material is found in the layers of theoptical stack or elsewhere in the multilayer optical film. As a result,if the refractive indices of only one of the materials making up theoptical stack is unknown and the refractive indices of the othermaterials in the optical stack are known, then measurement of therefractive indices of the optical stack will allow for calculation ofthe refractive indices of the unknown material. In some instances,measurement of the refractive indices may require destructive peeling orother known techniques of isolating the various layers of the multilayeroptical films.

Typically, the refractive indices of the birefringent materials in themultilayer optical film will be determined based on the above techniquesbecause it is the refractive indices of the birefringent materials thatchange in response to drawing or deformation. Assuming conservation ofmolecular polarizability within the birefringent materials of theoptical stack (an assumption that is typically considered a reasonableapproximation for many semi-crystalline polymers, including thepolyesters used in preferred underdrawn multilayer optical films, e.g.,PEN, PET and copolymers of PET and PEN), an anisotropic analogue of theClausius-Mossetti equation using a Lorenz-Lorentz local field yields thefollowing equation which results in a number referred to above as thetotal polarizability of the birefringent materials:(n ₁ ²−1)/(n ₁ ²+2)+(n ₂ ²−1)/(n ₂ ²+2)+(n ₃ ²−1)/(n ₃ ²+2)=ρK=Totalpolarizabilitywhere n₁, n₂ and n₃ are the refractive indices in the principaldirections of a given layer within the multilayer optical film, ρ is thedensity of the materials in that layer, and K is a volume polarizabilityper unit mass for the materials in that layer. Total polarizability is afunction of wavelength due to the wavelength dependence of the indicesof refraction. As a result, when referred to numerically herein, totalpolarizability will be determined with respect to light having awavelength of 632.8 nanometers (e.g., as provided by a helium-neon laserlight source).

It should be noted that an alternative to the total polarizabilityequation can also be used. In this alternative, each of the threeprincipal indices in the equation is set equal to the simple average ofthe three measured principal indices. The total polarizability is thencalled a refractivity and an analogous refractivity difference may bedefined. Likewise, density and crystallinity may be calculated. Thesemay vary from that calculated using the total polarizability. Fordiscussion purposes, the total polarizability calculation is used in theexamples that follow.

Many semi-crystalline polymers, such as isotactic polypropylene andpolybutylene terephthalate, are difficult to quench in the amorphousstate; or if quenched, are difficult to re-heat fast enough or processcold enough to prevent significant quiescent crystallization prior todrawing. Such polymers may not exhibit Regime I under typical processconditions. Rather, the connectivity in the morphology means that allsubsequent drawing is at least partially effective and the materialessentially begins in Regime II after casting and quenching. As withmaterials that exhibit Regime I behavior, these materials can still bedrawn and oriented. Moreover, the higher the degree of underdrawing(i.e. the lower the degree of drawing), the higher the level of residualextensibility available during the post processing (e.g. thermoforming).

From a functional standpoint, the onset of Regime II sets a certainlevel of extensibility related to the ultimate extensibility. Thisultimate extensibility will vary somewhat with draw conditions. Theamount of underdrawing is relative to this ultimate extensibility. Fullydrawn films are drawn near to this limit. Underdrawn films are drawnbelow this amount, but preferably have been drawn past the onset ofRegime II. The level of underdrawing desired may be a function of thelevel of extensibility desired for the subsequent post forming process.

The level of underdrawing is also a function of direction. Upon onset ofRegime II, a certain level of drawing is locked in. This amount may varyin direction depending upon the process conditions at the time of onset.For example, a uniaxially drawn film will have a higher degree ofunderdrawing in the non-drawn direction at the point of Regime II onset.In the case of mirror films, equal underdrawing in both directions maybe preferred. This may be achieved by minimizing the in-planebirefringence. As used here, the in-plane birefringence is simplydefined as the absolute value or magnitude of the difference between themaximum and minimum refractive index values in the plane on the film. Inthe case of a uniaxially drawn film, this is typically the differencebetween the indices of refraction in the draw and non-drawn directions.In the case of polarizing films, a large in-plane birefringence isdesired within the constraints of the underdrawing required to obtain adesired level of extensibility in the post process.

As can be seen by the directional nature of underdrawing, crystallinityor total polarizability alone does not fully characterize the level ofunderdrawing, although it sets useful limits with regards to thetransition between Regime I and II and between underdrawn and fullydrawn films. It should be understood that a certain level ofextensibility reflects a corresponding level of underdrawing. Forexample, films drawn quickly in Regime II may not achieve the same levelof crystallinity as those drawn slowly or those that continue to beheated at the draw temperature after drawing to heat set the films. Thelatter may be less extensible than the former; however, they may stillbe more extensible than other films slightly more drawn but less heatset. Thus maximum and minimum levels of crystallinity and/or totalpolarizability difference are most applicable in delineating the boundsof what is meant as an underdrawn film and not necessarily a solemeasure of the relative performance among that class of films.

The total polarizability difference of the birefringent materials inunderdrawn multilayer optical films including PEN (and, by thedefinitions provided below in the section regarding materials selection,predominantly PEN copolymers) as measured in the birefringent layers ispreferably within a range of from about 0.002 up to about 0.018, morepreferably within a range of from about 0.002 up to about 0.016. Withineither range, it may be desirable that the maximum in-planebirefringence of reflective polarizing multilayer optical films is lessthan about 0.22, more preferably less than about 0.17, and, in somecases, still more preferably less than about 0.15. In the case ofunderdrawn mirror films, a maximum in-plane birefringence of less thanabout 0.14 is preferred in combination with either of the ranges for thetotal polarizability difference in the birefringent materials.

Total polarizability difference of the birefringent materials inunderdrawn multilayer optical films including PET (and, by thedefinitions provided below in the section regarding materials selection,predominantly PET copolymers) as the measured birefringent layer ispreferably within a range of from about 0.002 up to about 0.030, morepreferably within a range of from about 0.002 up to about 0.0024. In thecase of mirror films, these ranges are preferably coupled with a maximumin-plane birefringence of less than about 0.11, more preferably lessthan about 0.04.

The differences between the preferred levels of total polarizability andbirefringence for the various polymers reflects the differences in theamorphous and crystalline densities of the different materials. Thedifferences also reflect the intrinsic maximum birefringence of thedifferent polymers, as well as the limits of extensibility after theonset of Regime II as discussed above.

In addition to the total polarizability and maximum in-planebirefringence, underdrawn multilayer optical films can also becharacterized by reflectivity. For example, where the totalpolarizability difference of the measured birefringent materials iswithin the various ranges discussed above, it may be preferred that themultilayer optical film reflect at least about 85% of normal incidentlight of desired wavelengths that is polarized along at least onein-plane axis, more preferably the film may reflect at least about 90%of normal incident light of desired wavelengths that is polarized alongat least one in-plane axis. If the multilayer optical film is intendedto be a mirror film, i.e., not a reflective polarizer, it may bepreferred that the reflective performance of the film in terms ofpercent reflectance hold for at least one of and more preferably twogenerally perpendicular in-plane axes.

As indicated in the equation presented above, total polarizability ofthe material(s) in a given layer of the optical stack of the multilayeroptical film represents the product of density and the volumepolarizability per unit mass of the material(s) in that layer. Thevolume polarizability per unit mass (K) is typically considered aninvariant material property under draw according to the conservation ofmolecular polarizability assumption discussed above. Drawing ofbirefringent materials causes strain-induced crystallization asdiscussed above and, in most birefringent materials, the density of thematerial varies based on whether the material is crystallized oramorphous.

As a result, the density of the birefringent materials in the multilayeroptical films changes based on the amount of strain-inducedcrystallization in the birefringent materials. Those changes in densitycan be used to estimate the level of strain-induced crystallization inthe underdrawn multilayer optical films according to the presentinvention. This method of determining the level of strain-inducedcrystallization is not, however, without its limits.

One class or type of preferred birefringent materials used in themultilayer optical films according to the present invention aresemi-crystalline. If the crystals in the semi-crystalline birefringentmaterials are relatively small, an effective refractive index for thesemi-crystalline aggregate may be measured. This is often the case inpolymers, such as polyesters (e.g., PEN and PET), that are drawn from arelatively amorphous state to a state of semi-crystallinity. In suchcases, the density of the birefringent material (based on the refractiveindices) may be estimated from the total polarizability and used todetermine the level of crystallinity in the birefringent materials usinga standard correlation between crystallinity and density.

In either case, the above discussions set out different approaches tocharacterizing underdrawn films according to the present invention. Inthe first, the strain-induced crystallinity of the birefringentmaterials is measured and used to define underdrawn multilayer opticalfilms. In the second, the refractive indices of the birefringentmaterials can be used to determine the total polarizability of thebirefringent materials which can also be used to define underdrawnmultilayer optical films. In still another manner, the strain-inducedcrystallinity can be determined based, at least in part, on therefractive indices used to determine total polarizability.

For example, the total polarizabilities of amorphous cast webs of PETand PEN are found to be about 0.989 and 1.083, respectively, and thedensities of the amorphous materials are measured using a standarddensity gradient column at about 1.336 and 1.329 grams per cubiccentimeter, respectively. The resulting volume polarizabilities can becalculated at about 0.740 and 0.815 cubic centimeters per gram for PETand PEN, respectively. Densities of drawn films of PET and PEN may nowbe calculated by dividing the total polarizabilities by the respectivevolume polarizabilities. Moreover, the crystallinity may be estimatedgiven the density of the pure crystalline phase, estimated as 1.407grams per cubic centimeter for the typical crystalline phase of PEN and1.455 grams per cubic centimeter for the crystalline PET.

The crystallinity can be estimated via a linear interpolation of theactual density between the amorphous density (zero crystallinity) andthe pure crystalline density. Such crystalline estimates may vary fromother measures as it neglects densification of the non-crystalline phasedue to orientation and rarefication of the crystalline phase due toimperfections and defects. Other methods for determining crystallinityinclude Differential Scanning calorimetry and X-ray Scattering.Measurements obtained by these methods may be correlated to the densityor total polarizability methods described herein through the use ofsuitable drawn film standards. It can typically be assumed thatcopolymers will have volume polarizabilities that are weight averages oftheir components, so that similar calculations can be made onco-polymers, if the type of crystals are known. Usually, this is thecrystal corresponding to the predominant crystallizing monomer orsubunit. Total polarizability can be used to characterize the underdrawnstate of many systems. However, lack of a definitive totalpolarizability measurement in no way limits the utility of theinvention. In some cases, the extensibility of a non-birefringent layermay be limiting. For example, a non-birefringent semi-crystalline secondmaterial layer may still become drawn during film processing. Underdrawing to suit this layer would be desirable When the material has verylow or no inherent birefringence, as is the case with a few polymerssuch as poly methyl methacrylate, then little or no orientationalinformation can be derived. Nevertheless, the extensibility of such anon-birefringent non-crystalline second material may also be limiting.In the case of non-crystalline materials, the orientation may be relaxedand thus the extensibility recovered by pre-heating prior to draw.Optimizing the conditions of such pre-heating must balance the recoveredextensibility of the amorphous material against any lost extensibilityby the birefringent semi-crystalline first material. In the examplesthat follow below, it is believed that the birefringent strain-hardeninglayers (e.g., PEN or 90/10 coPEN layers) are the limiting layers forextensibility, whereas the second material layers (e.g., PMMA, PETG, or70/0/30 coPEN) are believed to be nearly isotropic for the conditionsused to manufacture the optical stacks. Finally, in a semi-crystallinematerial, if the crystals are relatively large, haze and scattering mayobscure index measurements.

Process Considerations for Post-Forming Multilayer Optical Films

Because the post-formed multilayer optical films used in connection withthe present invention rely on birefringent materials that providestrain-induced refractive index differentials to obtain the desiredoptical properties, variations in deformation of the multilayer opticalfilm during post-forming can be particularly problematic.

As discussed above, the index of refraction differentials (Δx, Δy) inthe multilayer optical film as manufactured are, in large part, theresult of drawing of the multilayer optical film during manufacturingwhich causes the indices of refraction of the birefringent materials tochange. Those changes cause refractive index differentials large enoughto provide the desired reflective properties. Because the strain in themultilayer optical film during manufacturing is largely uniform, thestrain-induced index of refraction differentials are also largelyuniform over the film, and the resulting reflective properties are alsolargely uniform.

In post-forming processes the birefringent layers in the multilayeroptical film are subjected to additional strain. One difference frommanufacturing of the multilayer optical film is, however, that thestrain induced during post-forming is not uniform over the film. Thevariations in thickness of the optical stack in a post-formed multilayeroptical film as discussed above are, in part, indicative of thevariations in strain over the post-formed multilayer optical film.

As a result, if the birefringent materials in the multilayer opticalfilm are capable of further strain-induced index of refraction changes,the index of refraction differentials in the multilayer optical film maybe changed as a result of post-forming. Furthermore, if the straininduced during post-forming is not uniform, the index of refractionchanges in the post-formed multilayer optical film may also benon-uniform and may result in non-uniform optical properties in thepost-formed multilayer optical film.

In addition to non-uniform post-forming strain-induced changes, anotherdifficulty associated with post-forming multilayer optical filmsincluding strain-induced refractive index differentials in connectionwith birefringent materials is that many post-forming processes employheat to improve the working properties of the multilayer optical filmduring deformation. The strain-induced changes in the refractive indicesof the birefringent materials in the multilayer optical film aretypically the result of strain-induced crystallization of thebirefringent materials. The strain-induced crystallization andcorresponding refractive indices can, however, be changed when thebirefringent materials are subjected to heat during post-forming.

For example, heating may result in increased crystallization due to theheat during post-forming or decreased crystallization as a result ofmelting or relaxation during post-forming. In either case, changes inthe crystallization level of the birefringent materials can result in achange in the refractive index differentials in the film. The potentialcrystallization changes in the birefringent materials may be furtherexacerbated by the simultaneous post-forming deformation and heating ofthe film which, in combination, may cause greater changes in therecrystallization/refractive index of the birefringent materials thaneither action alone.

The present invention, however, overcomes these difficulties to providearticles including post-formed multilayer optical film and methods ofproducing those articles. These results are achieved even though all ofthe multilayer optical films referred to in connection with the presentinvention include birefringent materials and rely on strain-inducedrefractive index differentials to obtain the desired optical properties.

Although post-forming may be most advantageously pursued using the“underdrawn” multilayer optical films described above, it may also bepossible to obtain desirable post-forming results using multilayeroptical films including a birefringent material and other materials thatdo not meet the definitions of underdrawn multilayer optical films,e.g., constructed according to U.S. patent Ser. No. 08/472,241.

In the post-forming methods of the present invention, it may bedesirable to heat the multilayer optical films to forming temperaturesthat are near to, but below, the peak crystalline melting temperaturesof the birefringent materials. Such heating can improve theextensibility of multilayer optical films during post-formingprocessing. By heating the multilayer optical film to those levels, thetendency of the multilayer optical film to fracture or tear at a givendraw ratio during post-forming may be decreased. In addition, the forcesrequired to post-form the films may be reduced as a result of theincreased forming temperature.

Underdrawn multilayer optical films may also have increasedextensibility under these process conditions. Because processing underthese conditions is in the melting regime, precise temperature controlis desirable to ensure uniform drawing and reduce or prevent damage tothe post-formed multilayer optical film in the article. Such damagecould take the form of complete melting, with concomitant loss ofbirefringence and/or hole formation in the multilayer optical film.

Reducing the stress required for a given amount of deformation duringpost-forming may reduce the tendency of the materials in the film tofracture, thereby enhancing extensibility. Heating the multilayeroptical film to a forming temperature near the peak crystalline meltingtemperature of the birefringent material in the film may also enhanceextensibility by melting less perfect crystals, thereby loosening themorphological microstructure in the birefringent material layers.

For example, one material used in some preferred multilayer opticalfilms according to the present invention is polyethylene naphthalate(PEN), which has a peak melting point of about 270 degrees Celsius (520degrees Fahrenheit) using standard differential scanning calorimetry(DSC). The onset of melting is, however, often seen at about 255 degreesCelsius (490 degrees Fahrenheit) or below. This onset of melting may beattributable to the melting of less well-developed crystals within thePEN with the peak melting temperature being that point at which all ornearly all of the crystals in the material have melted. Heating thebirefringent materials in the multilayer optical film may also increasemobility within the microstructure, thereby activating crystal slip andother deformation mechanisms that could enhance extensibility of themultilayer optical film.

The extent to which heating may improve extensibility of the multilayeroptical films according to the present invention will, at least in part,vary based on the materials used in the films. Some materials mayexhibit larger increases in extensibility when heated as compared toothers. Furthermore, the combination of materials within each of themultilayer optical films according to the present invention can alsoaffect improvements in extensibility of the film as a whole.

For example, to improve the extensibility of the multilayer opticalfilms, it may be preferred to heat the multilayer optical films to aforming temperature in the range of from about 30 degrees Celsius (about55 degrees Fahrenheit) below the peak crystalline melting temperature ofthe birefringent material up to about the peak crystalline meltingtemperature of the birefringent material during post-forming. It may bemore preferred to heat the film to a forming temperature in the range offrom about 15 degrees Celsius (about 30 degrees Fahrenheit) below thepeak crystalline melting temperature of the birefringent material up toabout the peak crystalline melting temperature of the birefringentmaterial during post-forming. These forming temperatures may increaseextensibility and reduce the likelihood of fracture of multilayeroptical films during post-forming processing.

A way to improve uniformity in the multilayer optical film duringpost-forming is to include materials in the multilayer optical film thatare subject to strain hardening during deformation. Strain hardening isa property of materials in which the stress required to achieve aparticular level of strain increases as the material is strained (i.e.,stretched). Essentially, strain hardening materials may provideself-regulation of the thinning process due to post-forming.

In terms of molding, as the multilayer optical film is stretched duringpost-forming, unquenched sections of the film that have not yet madecontact with a mold surface will tend to draw more uniformly after theonset of strain hardening. As a result, those portions of the film thathave been stretched to the point at which strain hardening occurs willprogressively stretch less while those portions of the film that havenot experienced strain hardening will continue to stretch at fasterrates. The end result is that the thinner (i.e., strain hardened)portions of the film will thin to a certain point after which thethicker portions of the film will continue to stretch and becomethinner, effectively evening out the stretching or thinning of layers inthe multilayer optical film during post-forming processing. Thisreinforcement effect of strain hardening is also operative inpost-forming processes in which no mold is present to provide quenchingof the film during post-forming. One material that provides strainhardening properties in a multilayer optical film is PEN. In general,strain-hardening is typically observed in many semi-crystalline polymersat high enough levels of strain.

Strain-hardening can help to regulate the uniformity of the drawingprocess, thus potentially reducing variations in the amount ofdeformation experienced by the film during post-forming. If thebandwidth of the multilayer optical film as manufactured is specificallydesigned to the final biaxial draw ratio of the post-forming process,rather than the draw ratio at tear or fracture as discussed above, thenstrain hardening can allow the design of a multilayer optical film witha narrower, more reflective band for use in the post-forming process.

The effect of strain hardening may also influence the degree to whichvacuum-forming as one post-forming process will allow for adequate ordesirable mold replication. Pressurized or plug assisted moldingtechniques may be needed for accurate post-forming processing ofmaterials in which strain hardening potentially increases the resistanceof the film to stretching during the molding process. The effect ofstrain hardening may be influenced by both the post-forming drawconditions and the degree of draw (strain-hardening) before post-formingis initiated.

In addition to the above, one further consideration in developing anappropriate post-forming process may include an analysis of the rate ofcrystallization for the given materials as a function of temperature.Referring now to FIG. 13, an idealized graph of rate of crystallization(vertical axis) as a function of temperature (horizontal axis), it canbe seen that crystallization rate increases with temperature to acertain point, referred to as the peak crystallization rate temperatureT_(max), after which the rate of crystallization tends to fall again asthe temperature moves towards the peak crystalline melting temperatureT_(m) of the material. Differential scanning calorimetry may be used toestimate T_(max). For PEN, T_(max) has been estimated at about 220degrees Celsius (about 430 degrees Fahrenheit) using differentialscanning calorimetry upon heating at 20° C./min., and T_(max) has beenestimated at about 208 degrees Celsius (about 406 degrees Fahrenheit)using differential scanning calorimetry upon cooling at 5° C./min.Although we do not wish to be held to any theory, it is thought that theextensibility of multilayer optical films during post-forming can beimproved in many cases if the forming temperatures used are not the sameas the peak crystallization rate temperature of the birefringentmaterial or materials in the film. This may be particularly applicableto films that have not already been heat set, and especially underdrawnfilms. Nevertheless, if the film is sufficiently underdrawn,extensibility and thus post-processability may still be acceptable afterheating at these temperatures. The following discussion elucidates theeffects of post forming near T_(max) for some cases; e.g. certainunderdrawn, non-heatset films comprising certain polyesters. It shouldbe understood that multilayer optical films comprising materials otherthan polyesters may behave differently in their relation between peakcrystallization temperature and optimal forming temperatures.

Further crystallization and morphological changes during pre-heatingbefore post-forming may reduce extensibility and post-formability. Inone aspect, it may be preferred that the forming temperature of the filmduring post forming be lower than the peak crystallization ratetemperature of the birefringent material with the lowest peakcrystallization rate temperature in the film, more preferably more thanabout 10 degrees Celsius below the peak crystallization rate temperatureof the birefringent material with the lowest peak crystallization ratetemperature in the film, and even more preferably more than about 20degrees Celsius below the peak crystallization rate temperature of thebirefringent material with the lowest peak crystallization ratetemperature in the film. It may also be preferred that the formingtemperature be greater than the peak crystallization rate temperature ofthe birefringent material with the highest peak crystallization ratetemperature in the film, more preferably more than about 10 degreesCelsius above the peak crystallization rate temperature of thebirefringent material with the highest peak crystallization ratetemperature in the film, and even more preferably about 20 degreesCelsius above the peak crystallization rate temperature of thebirefringent material with the highest peak crystallization ratetemperature in the film.

These forming temperature limitations may be combined as desired. Forexample, it may be preferred that the forming temperature be more thanabout 10 degrees Celsius below the peak crystallization rate temperatureof the birefringent material with the lowest peak crystallization ratetemperature in the film or more than about 20 degrees Celsius above thepeak crystallization rate temperature of the birefringent material withthe highest peak crystallization rate temperature in the film. Inanother alternative, it may be desired that the forming temperature bemore than about 20 degrees Celsius below the peak crystallization ratetemperature of the birefringent material with the lowest peakcrystallization rate temperature in the film or greater than the peakcrystallization rate temperature of the birefringent material with thehighest peak crystallization rate temperature in the film. Othercombinations of these different limitations will also be apparent uponfurther analysis.

Where only one birefringent material is present in the multilayeroptical film, the forming temperature limitations can be more simplyexpressed. It may be preferred that the forming temperature of the filmbe different than the peak crystallization rate temperature of thebirefringent material in the film. Alternatively, it may be preferred todefine the forming temperature in terms of ranges, e.g., it may bepreferred that the forming temperature of the film be more than about 10degrees Celsius below the peak crystallization rate temperature of thebirefringent material, more preferably more than about 20 degreesCelsius below the peak crystallization rate temperature of thebirefringent material in the film. It may also be preferred that theforming temperature be more than about 10 degrees Celsius above the peakcrystallization rate temperature of the birefringent material film, morepreferably about 20 degrees Celsius above the peak crystallization ratetemperature of the birefringent material in the film.

After post-forming draw, it may be desirable to deliberately heat setthe formed article to improve its reflectivity. This heat settingpreferably follows the last post-forming drawing step; e.g., furthercrystallization can now be encouraged with attendant refractive indexdifference increases without consideration of further extensibilityafter the final post-forming draw step.

Although the methods of post-forming multilayer optical films in generalare discussed above, the post-forming of underdrawn multilayer opticalfilms may be varied while still providing desirable post-formingresults. One significant variation is that the forming temperature ofthe underdrawn multilayer optical films may lie well below the peakcrystallization rate temperatures of the birefringent materials withinthe films. Heat setting following the final post-forming draw step mayalso be desirable for articles manufactured from underdrawn multilayeroptical films. For example, the crystallinity (and, as a result, thereflectance) of portions of the underdrawn films that have not beendrawn during post-forming can be increased by heat-setting following thefinal post-forming draw steps. In addition, those portions of theunderdrawn film that were drawn during post-forming can also experienceincreased crystallinity and the attendant reflectance.

The underdrawn multilayer optical films can be provided with andpost-formed according to all of the variations described above withrespect to multilayer optical films in general. In other words, they canbe provided as highly reflective films that retain their reflectivityafter post-forming, etc. Furthermore, the modifications discussed abovefor thinning effects should also be considered when manufacturing andprocessing underdrawn multilayer optical films as well.

Post-Forming Selected Areas of Multilayer Optical Films

The articles including post-formed multilayer optical film and themethods of post-forming multilayer optical film described thus far havefocused on articles and methods in which the post-formed multilayeroptical film exhibits uniform optical properties. There are, howeverother articles and methods according to the present invention in whichit may be desirable to provide post-formed multilayer optical film withnon-uniform appearance. For example, it may be desired to providepost-formed multilayer optical film in which selected areas of themultilayer optical film are reflective for light of desired wavelengthswhile other selected areas of the post-formed multilayer optical filmtransmit light with the same or other desired wavelengths.

It may also be desirable to provide an article including post-formedmultilayer optical film in which selected areas in the post-formedmultilayer optical film are transmissive for visible wavelengths whilethe remainder of the post-formed multilayer optical film is reflectivefor visible wavelengths. To accomplish that result using a multilayeroptical film that is, as manufactured, reflective for visible light, themultilayer optical film in the selected areas could be stretched orthinned during the post-forming process such that all of the tunedbandwidths of the layers in the multilayer optical film stack in theselected transmissive areas are less than 400 nanometers afterpost-forming. The result of such a process would be an article includingpost-formed multilayer optical film that is highly reflective in theareas in which the reflective bandwidth remains in the visible spectrum,while the article would exhibit transmission in those areas in which thepost-formed multilayer optical film has been thinned to allowtransmission in the visible spectrum.

As an alternative to the previously described process, multilayeroptical films could be provided and post-formed in methods that resultin selected transmissive and reflective areas within the post-formedmultilayer optical film in the same article, but in which the unthinnedlayers remain transparent while those selected areas that are thinnedduring post-forming become reflective. For example, the multilayeroptical film as manufactured could be tuned to be reflective forwavelengths from about 900 to about 2025 nanometers, i.e., above thevisible spectrum. Films designed to reduce higher order harmonics thatgive perceptible color in the visible region of the spectrum may bepreferred. Some suitable films are described in U.S. Pat. Nos. Re.34,605 and 5,360,659, and in U.S. Pat. No. 6,207,260 (Wheatley et al.).

If such a multilayer optical film were post-formed, the selected areasof the multilayer optical film that are to be reflective would bedeliberately thinned during post-forming by an appropriate factor, e.g.,2.25, to retune the multilayer optical film in those selected areas suchthat visible wavelengths, i.e., those between about 400 to about 900nanometers, were substantially reflected. The remaining portions orareas of the multilayer optical film and the article that are notthinned sufficiently to reflect light in the visible spectrum wouldremain transmissive to visible light.

Many variations on these concepts can be envisioned. For example, themultilayer optical films could be post-formed in methods such that theselected areas are sharply defined resulting in short transition zonesbetween reflective/transparent areas, or they could be deliberatelydesigned with long transition zones in which the post-formed multilayeroptical film would exhibit iridescence as various wavelengths of lightwere reflected or transmitted. In another variation, different selectedareas could be thinned to reflect different selected wavelengths. Inthat manner, the selected areas could exhibit, e.g., different colors.The end result of applying the principles of multilayer optical filmsand methods of post-forming multilayer optical films according to thepresent invention is that desired combinations of optical effects can beobtained by selecting films with the desired optical and post-formingproperties and processing the films to obtain post-formed articles withthe desired optical properties.

One example of an article including post-formed multilayer optical filmthat is deformed in selected areas is depicted in FIG. 14. The article90 is a light box including a cover 92 that includes selected areas 94in the shape of indicia, in this case alphanumeric characters. In oneembodiment, the post-formed multilayer optical film of the cover 92 canbe formed from a multilayer optical film that is substantiallyreflective over the visible spectrum as manufactured. The multilayeroptical film can be post-formed in manners such as those described abovesuch that the multilayer optical film in the background area 96surrounding the selected areas 94 is thinned during post-forming suchthat the multilayer optical film in the background area 96 istransparent to at least a portion of the visible spectrum while theselected areas 94 are substantially unchanged.

In another embodiment, the background areas 96 may be maintained asreflective to the visible spectrum while the selected areas 94 aredeformed or thinned to provide a different optical effect from thebackground area 96. For example, the selected areas 94 may be embossedor blow molded or otherwise post-formed to thin the film in selectedareas 94 sufficiently that they become transmissive to at least aportion of the visible spectrum. Other variations on the constructionand manufacture of articles including post-formed multilayer opticalfilm in which selected areas are post-formed can also be envisionedbased on the examples discussed above.

Post-Forming Multilayer Optical Films with Substrates

FIG. 15 illustrates another feature of multilayer optical films andarticles including post-formed multilayer optical films according to thepresent invention. In some instances the post-formed multilayer opticalfilms alone may lack sufficient body or rigidity to provide the desiredmechanical properties. For example, the multilayer optical films maylack sufficient structural strength and/or stiffness to hold a desiredshape. FIG. 15 illustrates one solution to that problem in that themultilayer optical film 102 may be laminated to or otherwise attached toa substrate 104 to provide a composite 100 with the desired mechanicalproperties. In some instances, the substrate 104 may be manufacturedintegrally with the multilayer optical film 102, and in other cases themultilayer optical film 102 may be manufactured independently and laterattached to the substrate 104 to form the composite 100. If thesubstrate 104 is manufactured integrally with the multilayer opticalfilm 102, it may be a thicker layer of one of the materials provided inthe multilayer optical film 102 or it may be provided of anothermaterial that can be coextruded, cast, or otherwise formed with themultilayer optical film 102. In another alternative, the substrate 104may be provided as a coating on the multilayer optical film 102.

Furthermore, although a substrate 104 is shown only one side of themultilayer optical film 102, it will be understood that the substrate104 could be provided on both sides of the multilayer optical film 102.In addition, although the substrate 104 is depicted as a single layer,it will be understood that it could be a composite of different layersof the same or different materials based on the desired characteristicsof the substrate 104

In some cases, the materials selected for the substrate 104 may havelittle, if any, effect on the optical properties of the multilayeroptical film 102 but will provide a post-formable layer that isotherwise compatible with the multilayer optical film 102. In oneaspect, the substrate 104 may simply supply desired structuralstiffness/rigidity to the post-formed article, thereby reducing the needto laminate the post-formed multilayer optical film to anotherstructure. Examples of suitable materials for the substrate 104 include,but are not limited to polycarbonates, polyvinyl chlorides, PETG,acrylics, methacrylics, nylons, polyolefin, polypropylene, etc.

Another mechanical property that may be supplied by the substrate 104 isstrain-hardening during deformation as discussed above with respect tothe multilayer optical film. That strain-hardening property may be usedto limit the stresses placed on the attached multilayer optical film102, thereby acting to distribute the stresses over the multilayeroptical film 102 in a way that improves the post-formability of thecomposite 100 over the post-formability of the multilayer optical film102 alone.

The materials selected for substrate 104 may provide desired opticalproperties instead of, or in addition to, desired mechanical properties.For example, the substrate 104 may function as a mirror for selectedwavelengths of light such as infrared radiation, the substrate 104 mayinclude colorants or otherwise introduce color into the composite 100,the substrate 104 may provide diffusing properties in either or bothtransmittance or reflectance (to, e.g., reduce iridescence).

One class of films that may be particularly useful in connection withpost-forming of multilayer optical films is described in U.S. Pat. No.6,256,002.

Although in many instances the substrate 104 will be coextensive withthe multilayer optical film 102, it is also envisioned that thesubstrate may be attached only on selected areas of the multilayeroptical film as depicted in FIG. 16 where the substrate 114 is providedin selected areas on the multilayer optical film 112. It will also beunderstood that the substrate 114 may be provided in the form of a grid,mesh or other discontinuous form on the multilayer optical film 112 toimprove its post-formability. It may, for example, be advantageous toprovide the substrate 114 discontinuously in manners that assist indefining the selected areas of the post-formed multilayer optical filmas described above with respect to FIG. 14. In such an application, thesubstrate 114 may prevent or reduce drawing of the multilayer opticalfilm 112 during post-forming in manners that are difficult or impossibleto achieve through the use of post-forming techniques alone.

Regardless of whether the multilayer optical films used in connectionwith the present invention are included with substrates, underdrawn orfully drawn, etc. the selection of the materials in the films isdiscussed below.

Materials Selection

A variety of polymer materials suitable for use in the present inventionhave been taught for use in making coextruded multilayer optical films.For example, the polymer materials listed and described in U.S. Pat.Nos. 4,937,134, 5,103,337, 5,1225,448,404, 5,540,978, and 5,568,316 toSchrenk et al., and in U.S. Pat. Nos. 5,122,905, 5,122,906, and5,126,880 to Wheatley and Schrenk are useful for making multilayeroptical films according to the present invention. Of special interestare birefringent polymers such as those described in U.S. Pat. Nos.5,486,949 and 5,612,820 to Schrenk et al, U.S. Pat. No. 5,882,774 (Jonzaet al.), and U.S. application Ser. No. 09/006,601 (filed Jan. 13, 1998,now abandoned). Regarding the preferred materials from which the filmsare to be made, there are several conditions which should be met to makethe multilayer optical films of this invention. First, these filmsshould consist of at least two distinguishable polymers; the number isnot limited, and three or more polymers may be advantageously used inparticular films. Second, at least one of the two required polymers,referred to below as the first polymer, preferably has a stress opticalcoefficient having a large absolute value. In other words, it preferablyshould be capable of developing a large birefringence when stretched.Depending on the application, the birefringence may be developed betweentwo orthogonal directions in the plane of the film, between one or morein-plane directions and the direction perpendicular to the film plane,or a combination of these. In the special case that the isotropicindices are widely separated, the preference for large birefringence inthe first polymer may be relaxed, although at least some birefringenceis desired. Such special cases may arise in the selection of polymersfor mirror films and for polarizer films formed using a biaxial processwhich draws the film in two orthogonal in-plane directions. Third, thefirst polymer should be capable of maintaining birefringence afterstretching, so that the desired optical properties are imparted to thefinished film. Fourth, the other required polymer, referred to as the“second polymer”, should be chosen so that in the finished film, itsrefractive index, in at least one direction, differs significantly fromthe index of refraction of the first polymer in the same direction.Because polymeric materials are typically dispersive, that is, therefractive indices vary with wavelength, these conditions must beconsidered in terms of a particular spectral bandwidth of interest.

Other aspects of polymer selection depend on specific applications. Forpolarizing films, it is often advantageous for the difference in theindex of refraction of the first and second polymers in one film-planedirection to differ significantly in the finished film, while thedifference in the orthogonal film-plane index is minimized. If the firstpolymer has a large refractive index when isotropic, and is positivelybirefringent (that is, its refractive index increases in the directionof stretching), the second polymer will typically be chosen to have amatching refractive index, after processing, in the planar directionorthogonal to the stretching direction, and a refractive index in thedirection of stretching which is as low as possible. Conversely, if thefirst polymer has a small refractive index when isotropic, and isnegatively birefringent, the second polymer will typically be chosen tohave a matching refractive index, after processing, in the planardirection orthogonal to the stretching direction, and a refractive indexin the direction of stretching which is as high as possible.

Alternatively, it is possible to select a first polymer which ispositively birefringent and has an intermediate or low refractive indexwhen isotropic, or one which is negatively birefringent and has anintermediate or high refractive index when isotropic. In these cases,the second polymer may typically be chosen so that, after processing,its refractive index will match that of the first polymer in either thestretching direction or the planar direction orthogonal to stretching.Further, the second polymer will typically be chosen such that thedifference in index of refraction in the remaining planar direction ismaximized, regardless of whether this is best accomplished by a very lowor very high index of refraction in that direction.

One means of achieving this combination of planar index matching in onedirection and mismatching in the orthogonal direction is to select afirst polymer which develops significant birefringence when stretched,and a second polymer which develops little or no birefringence whenstretched, and to stretch the resulting film in only one planardirection. Alternatively, the second polymer may be selected from amongthose which develop birefringence in the sense opposite to that of thefirst polymer (negative-positive or positive-negative). Anotheralternative method is to select both first and second polymers which arecapable of developing birefringence when stretched, but to stretch intwo orthogonal planar directions, selecting process conditions, such astemperatures, stretch rates, post-stretch relaxation, and the like,which result in development of unequal levels of orientation in the twostretching directions for the first polymer, and/or for the secondpolymer such that one in-plane index is approximately matched to that ofthe first polymer, and the orthogonal in-plane index is significantlymismatched to that of the first polymer. For example, conditions may bechosen such that the first polymer has a biaxially oriented character inthe finished film, while the second polymer has a predominantlyuniaxially oriented character in the finished film.

The foregoing is meant to be exemplary, and it will be understood thatcombinations of these and other techniques may be employed to achievethe polarizing film goal of index mismatch in one in-plane direction andrelative index matching in the orthogonal planar direction.

Different considerations apply to a reflective, or mirror, film.Provided that the film is not meant to have some polarizing propertiesas well, refractive index criteria apply equally to any direction in thefilm plane, so it is typical for the indices for any given layer inorthogonal in-plane directions to be equal or nearly so. It isadvantageous, however, for the film-plane indices of the first polymerto differ as greatly as possible from the film-plane indices of thesecond polymer. For this reason, if the first polymer has a high indexof refraction when isotropic, it is advantageous that it also bepositively birefringent. Likewise, if the first polymer has a low indexof refraction when isotropic, it is advantageous that it also benegatively birefringent. The second polymer advantageously developslittle or no birefringence when stretched, or develops birefringence ofthe opposite sense (positive-negative or negative-positive), such thatits film-plane refractive indices differ as much as possible from thoseof the first polymer in the finished film. These criteria may becombined appropriately with those listed above for polarizing films if amirror film is meant to have some degree of polarizing properties aswell.

Colored films can be regarded as special cases of mirror and polarizingfilms. Thus, the same criteria outlined above apply. The perceived coloris a result of reflection or polarization over one or more specificbandwidths of the spectrum. The bandwidths over which a multilayer filmof the current invention is effective will be determined primarily bythe distribution of layer thicknesses employed in the optical stack(s),but consideration must also be given to the wavelength dependence, ordispersion, of the refractive indices of the first and second polymers.It will be understood that the same rules applied to the visiblespectrum will also generally be apply to the infrared and ultravioletwavelengths, as well as any other electromagnetic radiation for whichthe films are designed.

Absorbance is another consideration. For most applications, it isadvantageous for neither the first polymer nor the second polymer tohave any absorbance bands within the bandwidth of interest for the filmin question. Thus, all incident light within the bandwidth is eitherreflected or transmitted. However, for some applications, it may beuseful for one or both of the first and second polymer to absorbspecific wavelengths, either totally or in part.

Although many polymers may be chosen as the first polymer, certain ofthe polyesters have the capability for particularly large birefringence.Among these, polyethylene 2,6-naphthalate (PEN) is frequently chosen asa first polymer for films of the present invention. It has a very largepositive stress optical coefficient, retains birefringence effectivelyafter stretching, and has little or no absorbance within the visiblerange. It also has a large index of refraction in the isotropic state.Its refractive index for polarized incident light of 550 nm wavelengthincreases when the plane of polarization is parallel to the stretchdirection from about 1.64 to as high as about 1.9. Its birefringence canbe increased by increasing its molecular orientation which, in turn, maybe increased by stretching to greater stretch ratios with otherstretching conditions held fixed.

Other semicrystalline naphthalene dicarboxylic polyesters are alsosuitable as first polymers. Polybutylene 2,6-Naphthalate (PBN) is anexample. These polymers may be homopolymers or copolymers, provided thatthe use of comonomers does not substantially impair the stress opticalcoefficient or retention of birefringence after stretching. The term“PEN” herein will be understood to include copolymers of PEN meetingthese restrictions. In practice, these restrictions imposes an upperlimit on the comonomer content, the exact value of which will vary withthe choice of comonomer(s) employed. Some compromise in these propertiesmay be accepted, however, if comonomer incorporation results inimprovement of other properties. Such properties include but are notlimited to improved interlayer adhesion, lower melting point (resultingin lower extrusion temperature), better rheological matching to otherpolymers in the film, and advantageous shifts in the process window forstretching due to change in the glass transition temperature.

Suitable comonomers for use in PEN, PBN or the like may be of the diolor dicarboxylic acid or ester type. Dicarboxylic acid comonomers includebut are not limited to terephthalic acid, isophthalic acid, phthalicacid, all isomeric naphthalenedicarboxylic acids (2,6-, 1,2-, 1,3-,1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,7-, and 2,8-),bibenzoic acids such as 4,4′-biphenyl dicarboxylic acid and its isomers,trans-4,4′-stilbene dicarboxylic acid and its isomers, 4,4′-diphenylether dicarboxylic acid and its isomers, 4,4′-diphenylsulfonedicarboxylic acid and its isomers, 4,4′-benzophenone dicarboxylic acidand its isomers, halogenated aromatic dicarboxylic acids such as2-chloroterephthalic acid and 2,5-dichloroterephthalic acid, othersubstituted aromatic dicarboxylic acids such as tertiary butylisophthalic acid and sodium sulfonated isophthalic acid, cycloalkanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and itsisomers and 2,6-decahydronaphthalene dicarboxylic acid and its isomers,bi- or multi-cyclic dicarboxylic acids (such as the various isomericnorbornane and norbornene dicarboxylic acids, adamantane dicarboxylicacids, and bicyclo-octane dicarboxylic acids), alkane dicarboxylic acids(such as sebacic acid, adipic acid, oxalic acid, malonic acid, succinicacid, glutaric acid, azelaic acid, and dodecane dicarboxylic acid.), andany of the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Alternatively, alkyl esters of these monomers,such as dimethyl terephthalate, may be used.

Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,cycloalkane glycols such as 1,4-cyclohexanedimethanol and its isomersand 1,4-cyclohexanediol and its isomers, bi- or multicyclic diols (suchas the various isomeric tricyclodecane dimethanols, norbornanedimethanols, norbornene dimethanols, and bicyclo-octane dimethanols),aromatic glycols (such as 1,4-benzenedimethanol and its isomers,1,4-benzenediol and its isomers, bisphenols such as bisphenol A,2,2′-dihydroxy biphenyl and its isomers, 4,4′-dihydroxymethyl biphenyland its isomers, and 1,3-bis(2-hydroxyethoxy)benzene and its isomers),and lower alkyl ethers or diethers of these diols, such as dimethyl ordiethyl diols.

Tri- or polyfunctional comonomers, which can serve to impart a branchedstructure to the polyester molecules, can also be used. They may be ofeither the carboxylic acid, ester, hydroxy or ether types. Examplesinclude, but are not limited to, trimellitic acid and its esters,trimethylol propane, and pentaerythritol.

Also suitable as comonomers are monomers of mixed functionality,including hydroxycarboxylic acids such as parahydroxybenzoic acid and6-hydroxy-2-naphthalenecarboxylic acid, and their isomers, and tri- orpolyfunctional comonomers of mixed functionality such as5-hydroxyisophthalic acid and the like.

Polyethylene terephthalate (PET) is another material that exhibits asignificant positive stress optical coefficient, retains birefringenceeffectively after stretching, and has little or no absorbance within thevisible range. Thus, it and its high PET-content copolymers employingcomonomers listed above may also be used as first polymers in someapplications of the current invention. The term “PET” as used hereinwill be understood to include PET and its high PET content copolymersthat function similarly to PET alone.

When a naphthalene dicarboxylic polyester such as PEN or PBN is chosenas first polymer, there are several approaches which may be taken to theselection of a second polymer. One preferred approach for someapplications is to select a naphthalene dicarboxylic copolyester (coPEN)formulated so as to develop significantly less or no birefringence whenstretched. This can be accomplished by choosing comonomers and theirconcentrations in the copolymer such that crystallizability of the coPENis eliminated or greatly reduced. One typical formulation employs as thedicarboxylic acid or ester components dimethyl naphthalate at from about20 mole percent to about 80 mole percent and dimethyl terephthalate ordimethyl isophthalate at from about 20 mole percent to about 80 molepercent, and employs ethylene glycol as diol component. Of course, thecorresponding dicarboxylic acids may be used instead of the esters. Thenumber of comonomers which can be employed in the formulation of a coPENsecond polymer is not limited. Suitable comonomers for a coPEN secondpolymer include but are not limited to all of the comonomers listedabove as suitable PEN comonomers, including the acid, ester, hydroxy,ether, tri- or polyfunctional, and mixed functionality types.

Often it is useful to predict the isotropic refractive index of a coPENsecond polymer. A volume average of the refractive indices of themonomers to be employed has been found to be a suitable guide. Similartechniques well-known in the art can be used to estimate glasstransition temperatures for coPEN second polymers from the glasstransitions of the homopolymers of the monomers to be employed.

In addition, polycarbonates having a glass transition temperaturecompatible with that of PEN and having a refractive index similar to theisotropic refractive index of PEN are also useful as second polymers.Polyesters, copolyesters, polycarbonates, and copolycarbonates may alsobe fed together to an extruder and transesterified into new suitablecopolymeric second polymers.

It is not required that the second polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylates,acetates, and methacrylates may be employed. Condensation polymers otherthan polyesters and polycarbonates may also be used. Examples include:polysulfones, polyamides, polyurethanes, polyamic acids, and polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful for increasing the refractive index of the second polymer to adesired level. Acrylate groups and fluorine are particularly useful indecreasing refractive index when this is desired.

It will be understood from the foregoing discussion that the choice of asecond polymer is dependent not only on the intended application of themultilayer optical film in question, but also on the choice made for thefirst polymer, and the processing conditions employed in stretching.Suitable second polymer materials include but are not limited topolyethylene naphthalate (PEN) and isomers thereof (such as 2,6-, 1,4-,1,5-, 2,7-, and 2,3-PEN), polyalkylene terephthalates (such aspolyethylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), other polyesters,polycarbonates, polyarylates, polyamides (such as nylon 6, nylon 11,nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10, nylon 6/12, andnylon 6/T), polyimides (including thermoplastic polyimides andpolyacrylic imides), polyamide-imides, polyether-amides,polyetherimides, polyaryl ethers (such as polyphenylene ether and thering-substituted polyphenylene oxides), polyarylether ketones such aspolyetheretherketone (“PEEK”), aliphatic polyketones (such as copolymersand terpolymers of ethylene and/or propylene with carbon dioxide),polyphenylene sulfide, polysulfones (including polyethersulfones andpolyaryl sulfones), atactic polystyrene, syndiotactic polystyrene(“sPS”) and its derivatives (such as syndiotactic poly-alpha-methylstyrene and syndiotactic polydichlorostyrene), blends of any of thesepolystyrenes (with each other or with other polymers, such aspolyphenylene oxides), copolymers of any of these polystyrenes (such asstyrene-butadiene copolymers, styrene-acrylonitrile copolymers, andacrylonitrile-butadiene-styrene terpolymers), polyacrylates (such aspolymethyl acrylate, polyethyl acrylate, and polybutyl acrylate),polymethacrylates (such as polymethyl methacrylate, polyethylmethacrylate, polypropyl methacrylate, and polyisobutyl methacrylate),cellulose derivatives (such as ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (such as polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers and copolymers (such as polytetrafluoroethylene,polytrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride,fluorinated ethylene-propylene copolymers, perfluoroalkoxy resins,polychlorotrifluoroethylene, polyethylene-co-trifluoroethylene,polyethylene-co-chlorotrifluoroethylene), chlorinated polymers (such aspolyvinylidene chloride and polyvinyl chloride), polyacrylonitrile,polyvinylacetate, polyethers (such as polyoxymethylene and polyethyleneoxide), ionomeric resins, elastomers (such as polybutadiene,polyisoprene, and neoprene), silicone resins, epoxy resins, andpolyurethanes.

Also suitable are copolymers, such as the copolymers of PEN discussedabove as well as any other non-naphthalene group-containing copolyesterswhich may be formulated from the above lists of suitable polyestercomonomers for PEN. In some applications, especially when PET serves asthe first polymer, copolyesters based on PET and comonomers from saidlists above (coPETs) are especially suitable. In addition, either firstor second polymers may consist of miscible or immiscible blends of twoor more of the above-described polymers or copolymers (such as blends ofsPS and atactic polystyrene, or of PEN and sPS). The coPENs and coPETsdescribed may be synthesized directly, or may be formulated as a blendof pellets where at least one component is a polymer based onnaphthalene dicarboxylic acid or terephthalic acid and other componentsare polycarbonates or other polyesters, such as a PET, a PEN, a coPET,or a co-PEN.

Another preferred family of materials for the second polymer for someapplications are the syndiotactic vinyl aromatic polymers, such assyndiotactic polystyrene. Syndiotactic vinyl aromatic polymers useful inthe current invention include poly(styrene), poly(alkyl styrene)s, poly(aryl styrene)s, poly(styrene halide)s, poly(alkoxy styrene)s,poly(vinyl ester benzoate), poly(vinyl naphthalene), poly(vinylstyrene),and poly(acenaphthalene), as well as the hydrogenated polymers andmixtures or copolymers containing these structural units. Examples ofpoly(alkyl styrene)s include the isomers of the following: poly(methylstyrene), poly(ethyl styrene), poly(propyl styrene), and poly(butylstyrene). Examples of poly(aryl styrene)s include the isomers ofpoly(phenyl styrene). As for the poly(styrene halide)s, examples includethe isomers of the following: poly(chlorostyrene), poly(bromostyrene),and poly(fluorostyrene). Examples of poly(alkoxy styrene)s include theisomers of the following: poly(methoxy styrene) and poly(ethoxystyrene). Among these examples, particularly preferable styrene grouppolymers, are: polystyrene, poly(p-methyl styrene), poly(m-methylstyrene), poly(p-tertiary butyl styrene), poly(p-chlorostyrene),poly(m-chloro styrene), poly(p-fluoro styrene), and copolymers ofstyrene and p-methyl styrene.

Furthermore, comonomers may be used to make syndiotactic vinyl aromaticgroup copolymers. In addition to the monomers for the homopolymerslisted above in defining the syndiotactic vinyl aromatic polymers group,suitable comonomers include olefin monomers (such as ethylene,propylene, butenes, pentenes, hexenes, octenes or decenes), dienemonomers (such as butadiene and isoprene), and polar vinyl monomers(such as cyclic diene monomers, methyl methacrylate, maleic acidanhydride, or acrylonitrile).

The syndiotactic vinyl aromatic copolymers of the present invention maybe block copolymers, random copolymers, or alternating copolymers.

The syndiotactic vinyl aromatic polymers and copolymers referred to inthis invention generally have syndiotacticity of higher than 75% ormore, as determined by carbon-13 nuclear magnetic resonance. Preferably,the degree of syndiotacticity is higher than 85% racemic diad, or higherthan 30%, or more preferably, higher than 50%, racemic pentad.

In addition, although there are no particular restrictions regarding themolecular weight of these syndiotactic vinyl aromatic polymers andcopolymers, preferably, the weight average molecular weight is greaterthan 10,000 and less than 1,000,000, and more preferably, greater than50,000 and less than 800,000.

The syndiotactic vinyl aromatic polymers and copolymers may also be usedin the form of polymer blends with, for instance, vinyl aromatic grouppolymers with atactic structures, vinyl aromatic group polymers withisotactic structures, and any other polymers that are miscible with thevinyl aromatic polymers. For example, polyphenylene ethers show goodmiscibility with many of the previous described vinyl aromatic grouppolymers.

When a polarizing film is made using a process with predominantlyuniaxial stretching, particularly preferred combinations of polymers foroptical layers include PEN/coPEN, PET/coPET, PEN/sPS, PET/sPS,PEN/Eastar,™ and PET/Eastar,™ where “coPEN” refers to a copolymer orblend based upon naphthalene dicarboxylic acid (as described above) andEastar™ is a polyester or copolyester (believed to comprisecyclohexanedimethylene diol units and terephthalate units) commerciallyavailable from Eastman Chemical Co. When a polarizing film is to be madeby manipulating the process conditions of a biaxial stretching process,particularly preferred combinations of polymers for optical layersinclude PEN/coPEN, PEN/PET, PEN/PBT, PEN/PETG and PEN/PETcoPBT, where“PBT” refers to polybutylene terephthalate, “PETG” refers to a copolymerof PET employing a second glycol (usually cyclohexanedimethanol), and“PETcoPBT” refers to a copolyester of terephthalic acid or an esterthereof with a mixture of ethylene glycol and 1,4-butanediol.

Particularly preferred combinations of polymers for optical layers inthe case of mirrors or colored films include PEN/PMMA, PET/PMMA,PEN/Ecdel,™ PET/Ecdel,™ PEN/sPS, PET/sPS, PEN/coPET, PEN/PETG, andPEN/THV,™ where “PMMA” refers to polymethyl methacrylate, Ecdel™ is athermoplastic polyester or copolyester (believed to comprisecyclohexanedicarboxylate units, polytetramethylene ether glycol units,and cyclohexanedimethanol units) commercially available from EastmanChemical Co., “coPET” refers to a copolymer or blend based uponterephthalic acid (as described above), “PETG” refers to a copolymer ofPET employing a second glycol (usually cyclohexanedimethanol), and THV™is a fluoropolymer commercially available from 3M Co.

For mirror films, a match of the refractive indices of the first polymerand second polymer in the direction normal to the film plane issometimes preferred, because it provides for constant reflectance withrespect to the angle of incident light (that is, there is no Brewster'sangle). For example, at a specific wavelength, the in-plane refractiveindices might be 1.76 for biaxially oriented PEN, while the filmplane-normal refractive index might fall to 1.49. When PMMA is used asthe second polymer in the multilayer construction, its refractive indexat the same wavelength, in all three directions, might be 1.495. Anotherexample is the PET/Ecdel™ system, in which the analogous indices mightbe 1.66 and 1.51 for PET, while the isotropic index of Ecdel™ might be1.52.

It is sometimes preferred for the multilayer optical films of thecurrent invention to consist of more than two distinguishable polymers.A third or subsequent polymer might be fruitfully employed as anadhesion-promoting layer between the first polymer and the secondpolymer within an optical stack, as an additional component in a stackfor optical purposes, as a protective boundary layer between opticalstacks, as a skin layer, as a functional coating, or for any otherpurpose. As such, the composition of a third or subsequent polymer, ifany, is not limited. Some preferred multicomponent constructions aredescribed in U.S. Pat. No. 6,207,260 (Wheatley et al.).

The selection criteria for the materials of the optical stack layers mayalso be useful in the selection of appropriate materials for thickinternal or external skin protective layers. The criteria for the secondpolymer may be more desirable than those for the first polymer. In somecases, however, the mechanical properties of the birefringent firstmaterial, such as high glass transition temperature to reduce stickingto rollers, low coefficients of thermal expansion, mechanical stiffness,etc., may be desirable. In the case of films designed for post-forming,it may be desirable to use materials of lower draw stiffness to improveformability at a given applied stress, e.g., vacuum pressure, orotherwise improve extensibility.

EXAMPLES

Advantages of the invention are illustrated by the following examples.However, the particular materials and amounts thereof recited in theseexamples, as well as other conditions and details, are to be interpretedto apply broadly in the art and should not be construed to unduly limitthe invention.

Example 1 Fully Drawn Mirror Film

A multilayer film of polyethylene 2,6-naphthalate (PEN) andpolymethylmethacrylate (PMMA) was co-extruded, cast and drawn to make afully drawn PEN:PMMA multilayer mirror film. A 0.48 IV PEN (made by 3MCo., St. Paul Minn.) was dried at 135° C. for 24 hours and then feddirectly into a single screw extruder with an exit temperature of about285° C. PMMA (CP-82 grade available from Ashland Chemical) was dried byfeeding into a twin screw extruder equipped with a vacuum and with anexit temperature of about 260° C. The resin streams were co-extrudedinto a 224 multilayer feedblock set at 275° C. and equipped with aninternal protective boundary layer (PBL). Pumping rates were maintainedso that the approximate optical thickness of each PEN:PMMA layer pairwas approximately equal in the optical stack, i.e. an “f-ratio” of 0.5.The PBL was supplied with PEN at approximately one-half the volume asthat supplied to the sum of all the PEN layers in the optical stack. Thelayer pairs in the optical stack had an approximately linear gradient inoptical thickness. The multilayer stack including the PBL was split withan asymmetric multiplier to form two streams in a width ratio of 1.55:1,spread to equivalent widths and re-stacked to form a two packetmultilayer stack of 448 layers separated by an internal protectivelayer. An additional PEN (IV 0.48) skin was added to each side of themultilayer stack, with each skin layer comprising about 10% of the totalvolumetric flow. The total stream was cast from a die at about 285 Conto a quench wheel set at 65° C. The PEN skins refractive indices wereessentially isotropic after casting with indices of 1.64 at 632.8 nm asmeasured by a Metricon Prism Coupler, available from Metricon,Piscataway, N.J. The cast thickness was approximately 0.07 cm.

The first draw process used a conventional length orienter (LO). Thefilm was preheated with hot rollers set at 125 C and fed into a draw gapcomprising a slow roll and fast roll and an infra-red heater set at 80%power. The infrared heater consisted of an assembly of IR heaterelements (approximately 5000 watts per element), each about 65 cm long.The elements were approximately 10 cm above the film. Residence time inthe draw gap was about 4 seconds. The fast roll was set to accomplish a3.3 times draw and the drawn film was quenched. The average PEN indiceswere highly oriented at about 1.79, 1.59 and 1.55 as measured by theMetricon Prism Coupler) in the in-plane draw direction y-axis (MD), thein-plane crossweb direction x-axis (TD) and thickness (z) (ND)direction, respectively. The film was next drawn transversely using aconventional tenter in a second draw step to a final transverse drawratio of about 4.0. The tenter was set at 132° C. in the preheat, 135°C. in the draw zone, 249° C. in the heat set zone and 49° C. in thequench zone. Preheating, drawing and heat setting were accomplished overperiods of approximately 25, 5 and 40 seconds. The final PEN indiceswere 1.7284, 1.7585 and 1.5016 while the PMMA indices were approximatelyisotropic at 1.49, all at 632.8 nm as measured by the Metricon PrismCoupler. The measured reflectance band covered the spectrum from 400 nmto 950 nm with over 95% average reflectivity. The total polarizabilitywas thus calculated as 1.1043 and the total polarizability differencewas 0.0215 for the birefringent PEN layer. The density was estimated as1.3549 g/cc as discussed above and the fractional crystallinity wascalculated as 0.33.

Example 2 Underdrawn Mirror Film

A multilayer film of PEN and PETG (a copolymer of PET comprising somesubstitution of ethylene glycol with 1,4 cyclohexane diol duringpolymerization) was co-extruded, cast and drawn to make an underdrawnPEN:PETG multilayer mirror film. A 0.48 IV PEN (made by 3M Co., St. PaulMinn.) was dried at 135° C. for 24 hours and then fed directly into asingle screw extruder with exit temperature about 285° C. PETG(available from Eastman Chemical, TN) was dried by feeding into a twinscrew extruder equipped with a vacuum and with an exit temperature ofabout 285° C. These resin streams were co-extruded into a 209-multilayerfeedblock set at 285° C. Pumping rates were maintained so that theapproximate optical thickness of each PEN:PETG layer pair wasapproximately equal in the optical stack, i.e. an “f-ratio” of 0.5. Thelayer pairs in the optical stack had an approximately linear gradient inoptical thickness. A PBL was then supplied with PEN in an amountapproximately 20% of the final volumetric flow. The multilayer stackincluding the PBL was split with an asymmetric multiplier to form twostreams in width ratio of 1.55:1, spread to equivalent widths andre-stacked to form a two packet multilayer stack of 418 optical layersseparated by an internal protective layer. An additional PEN (IV 0.48)skin was added to each side of the multilayer stack, each skin layercomprising about 12.5% of the total volumetric flow. The total streamwas cast from a die at about 285 C onto a quench wheel set at 65° C. ThePEN skins refractive indices were essentially isotropic after castingwith indices of 1.64 at 632.8 nm as measured by the Metricon PrismCoupler. The cast thickness was approximately 0.07 cm.

The first draw process used a conventional length orienter (LO). Thefilm was preheated with hot rollers set at 120° C. and fed into a drawgap comprising a slow roll and fast roll and an infrared heater set at60% power. The infrared heater consisted of an assembly of IR heaterelements (approximately 5000 watts per element), each about 65 cm long.The elements were approximately 10 cm above the film. Residence time inthe draw gap was about 4 seconds. The fast roll was set to accomplish a2.7 times draw and the drawn film was quenched. The film was next drawntransversely using a conventional tenter in a second draw step to afinal transverse draw ratio of about 3.3. The tenter was set at 132° C.in the preheat zone, 135° C. in the draw zone, 135 vC in the heat setzone and 49 C in the quench zone. Preheating, drawing and heat settingwere accomplished over periods of approximately 25, 5 and 40 seconds.The final PEN indices were 1.69, 1.72 and 1.53 while the PETG indiceswere approximately isotropic at 1.56, all at 632.8 nm as measured by theMetricon Prism Coupler. Note that PMMA could be substituted for the PETGin this example with improved optical performance.

The film, made as described, is an underdrawn mirror film. This film wasre-drawn simultaneously at 135° C. over 1 second to an additionalmeasured true draw ratios of 1.27×1.22, with a biaxial draw ratio ofabout 1.55, as might occur during a thermoforming process. The same filmwas then further heat set for 4 minutes at 175° C. to form a fully drawnfilm. Shorter time periods, e.g. several seconds, could be applied athigher temperatures, e.g. 220° C., to accomplish similar heat setresults. The underdrawn film had high extensibility. In another case,the underdrawn film was re-drawn simultaneously at 135° C. over 2.4seconds to a measured true draw ratios of 1.63×1.58, i.e. the biaxialdraw ratio during re-drawing was 2.6. The progress of index (n)development in the MD, TD and ND directions (x,y,z directions) at 632.8nm as well as the calculated total polarizability (TP), totalpolarizability difference (TPD), estimated density (in g/cc) andfractional crystallinity (X) (calculated from the density) are presentedin the following table:

Case MD n TD n ND n TP TPD Density X Under- 1.6949 1.7283 1.5275 1.09040.0077 1.3379 0.1113 drawn Re-drawn 1.7033 1.7331 1.5168 1.0908 0.00801.3383 0.1167 1.27 × 1.22 Re-drawn 1.7124 1.7302 1.5081 1.0891 0.00641.3363 0.0907 1.63 × 1.58 Heat set 1.7188 1.7511 1.4995 1.0962 0.01351.345 0.2021In this series of examples, re-drawing to the higher biaxial draw ratiodoes not greatly alter the crystallinity or total polarizability.

FIGS. 17 and 18 present the measured transmissions of light polarized inthe MD and TD directions using a Perkin-Elmer Lambda-19. Outside of themultilayer reflection band, the transmission is about 85% rather than100% due to surface reflections. The following table identifies some ofthe approximate spectral features:

Ave. % Approx. Trans- % Min. Location Approx. mission Transmission ofMin. Case Band MD TD MD TD approx. Under- 725-1425 nm  26.2 14.7 1.8 0.51390 nm  drawn Re-drawn 430-920 nm 19.0 12.0 3.1 2.2 890 nm 1.27 × 1.22Heat set 420-950 nm 15.6 10.0 1.1 <0.3 855 nm

The band is primarily the first order reflection band, although somesecond order reflections may also contribute to this band. Higher orderpeaks are evident as well, such as the third order peak at about 450 nmfor the 1390 nm reflection peak (i.e. transmission valley). The bandshifts in proportion to the biaxial draw ratio as expect between theunderdrawn and re-drawn case. The band transmission decreases, i.e. theband reflectivity increases, after heat setting as a result of increasedindex differences between the birefringent PEN layers and theapproximately isotropic PETG layers.

Example 3 Comparison of Fully Drawn, Underdrawn, and Cast Web Films

A fully drawn film made according to example 1, an underdrawn film madeaccording to example 2 and an undrawn cast web made in a similar fashionto that in example 1 substituting a copolymer of PEN for the PEN layersand using thinner skins and PBL layers, were thermoformed intoapproximately spherical caps using the process described below The fullydrawn film was a multilayer optical mirror film comprising approximately400 optical layers alternating in PEN and PMMA with thicker PEN skinlayers and a thick internal PEN layer, originally drawn 3.3×4.0. Theunderdrawn film was a multilayer optical mirror comprising approximately400 optical layers alternating in PEN and PETG (a copolymer of PET) withthicker PEN skin layers and a thick internal PEN layer, originally drawnabout 80% of the fully drawn film, i.e. 2.7×3.3, under similar processconditions of applied heating and line speed (e.g. strain rate) on thesame process line. The cast web comprised approximately 400 layersalternating in a coPEN consisting of 90% PEN and 10% PET subunits (i.e.a 90/10 coPEN) and PMMA with thicker 90/10 coPEN skin layers and a thickinternal 90/10 coPEN layer. The films were place over a circularaperture about 3.3 cm in diameter. A vacuum of nearly one atmosphere wasapplied and the films were heated for a few seconds using a heat gun.The temperature was estimated at about 200° C., using a thermocoupleplaced in the air stream of the heat gun at the same distance andresidence time as the film.

The cast web drew the most but also drew the most unevenly, forming anelongated, roughly hemispherical cap. The base of the cap had an outerdiameter of 3.2 cm. The height of the cap was about 1.75 cm. The castweb was originally about 675 microns thick. Near the top of the cap, thethickness varied between 140 and 225 microns. The biaxial draw ratiothus varied widely with a maximum value of around 4.8. The initialrefractive index in the 90/10 coPEN skin layer was nearly isotropic,with a value of 1.6355 at 632.8 nm. At the thinnest part, the indices inthe three principal directions in the final cap were approximately1.6685, 1.6766 and 1.5784 at 632.8 nm.

The fully drawn mirror film and the underdrawn mirror films drew muchmore uniformly with a spread in thickness of about 10% or less acrossmost of the approximately spherical cap, as would be expected withstrain-hardening films. The fully drawn film was initially 68 micronsand thinned to about 58 microns across the cap, giving a biaxial drawratio of about 1.17. The base of the cap had an outer diameter of 3.25cm. The height of the cap was about 0.55 cm. The indices of refractionin the birefringent PEN skin layer, initially at 1.7276, 1.7693 and1.5014, remained about the same after thermoforming. The film remainedhighly reflective. The underdrawn film was initially 105 microns andthinned to about 78 microns across the cap, giving a biaxial draw ratioof about 1.35. The base of the cap had an outer diameter of 3.25 cm. Theheight of the cap was about 0.65 cm. The indices of refraction in thebirefringent PEN skin layer, initially at 1.6939, 1.7367 and 1.5265,increased slightly in the originally in-plane directions to 1.7120 and1.7467 while the thickness direction index decreased to 1.5081 afterthermoforming. In this particular case, the initial underdrawn film wastransparent at the lower spectral end of the visible wavelengths due toits increased thickness relative to the fully drawn film. Thereflectivity across the visible spectrum increased in the spherical capdue to the band shifting to cover these lower wavelengths as well as theincrease in index difference between the birefringent PEN and the nearlyisotropic PETG layers.

Comparative Example 1 Thermoformed Cast Web

A cast web was about 34.5 mils thick was made as described in Example 1.The cast web as described in Example 3 was heated and vacuum formed intoa deep cylindrical mold. The resulting part formed had a cylindricalshaft and a spherical end cap. The inner diameter of the cylinder wasabout 2.1 cm. The depth of the cylinder and spherical cap was about 1.9cm. The deviation from the straight sides of the cylinder into the capoccurs at about 1 cm, so that the cap is nearly hemispherical. A gridwas drawn on the part before forming with each line separated by about0.6 cm.

Large nonuniformities in draw conditions were observed across thesample. Over the top of the cap, a grid segment was stretched to about2.8 cm, suggesting a nominal draw over the hemisphere of about 4.7×4.7,resulting in a biaxial draw ratio of 22. Uniform drawing across theentire shaped part above the base would have required a biaxial drawratio of about 4. There were signs of severe delamination failure in thecast web. This became a benefit for the analysis: to further analyze thepart, the skin layer interior to the part was stripped off with theremainder of the piece remaining intact. Five samples were cut from theskin as shown in the table below:

thickness thickness In-plane In-plane Z biax draw Total Estimated Sample(min.) (max.) Index, nx index, ny Index, nz ratio PolarizabilityCrystallinity 1-base 3.67 3.75 1.6435 1.6419 1.6429  1.0 1.330743 0.01862-top 0.12 0.14 1.7293 1.7067 1.5419 28.5 1.353209 0.3077 3-cyl 2.553.24 1.6572 1.6431 1.6275 1.45-1.15 1.3284 −0.012 4-cyl 1.25 1.76 1.66861.6395 1.6195 2.97-2.11 1.330097 0.0103 5-top 0.12 0.14 1.7190 1.70301.5557 28.5 1.354125 0.3195 Thickness are measured in mils (0.001inches). All optical measurements were taken at 632.8 nm using theMetricon Prism Coupler.

Sample #1 shows that the undrawn skin layer is about 11% of the totalthickness of the cast web. Because of delamination, the base was onlymeasured where this was not present. The biaxial draw ratio was thencalculated using the ratio of this average base thickness to the finalsample thickness.

Samples #2 and #3 were essentially at the top of the spherical cap. Thetrue biaxial draw ratio is slightly higher than that anticipated by thegridline expansion as would be expected for a nonuniformly drawn piece:the cap is thinnest at the top. The thickness was determined both usinga caliper gauge and using the thin film thickness calculation availableon the Metricon. The latter yielded a value of 3.5 microns, that is,about 0.14 mils, in agreement with the caliper gauge. Note that the“in-plane” indices are less than other fully drawn mirror films, thehigh total polarizability resulting from the high z indices.

Sample #3 was taken from the bottom of the cylinder, from about 0.2 to0.7 cm above the base. The long direction was cut around thecircumference of the cylinder. This circumferential direction isconsidered the x direction for purposes of the preceding table. Sample#4 was cut directly above, from about 0.7 to 1.0 cm above the base.Apparently, the draw is more directed around the hoop of the cylinderthan towards the cap as indicated by the indices of refraction. The lowbiaxial draw ratios lead to very low deviation from isotropy in thissample.

The effectiveness of the orientation process can also be seen byestimating the crystallinity using the concept of total polarizability.Due to experimental error, the estimates are only good to about +/−0.02fractional crystallinity as defined here using the total polarizabilityconcept. From the values indicated in the table, the base and cylinderwall sections were still essentially amorphous: only the highly drawnspherical cap had significant crystallinity. Besides the concomitanteffects on reflectivity via the index differences, this non-uniformityalso results in non-uniform mechanical properties of the formed part.

Example 4 Relative Extensibility of Fully Drawn and Underdrawn Films

The relative extensibility of a fully drawn film made in accordance withexample 1 was compared to that of an underdrawn film made in accordancewith example WM2. The initial biaxial draw ratio of the fully drawn filmwas 13.2 (3.3×4.0), while the initial biaxial draw ratio of theunderdrawn film was 8.9 (2.7×3.3). Again, the draw conditions used tomake these films were similar, except for the final draw ratios in eachdirection. Several samples of each were drawn simultaneously biaxiallyat an initial rate of 10%/second (e.g. 1.5×1.5 over 5 seconds) at 130°C. and 160° C. until breakage. A biaxial laboratory film stretcher wasused, in which the film is gripped by pressure actuated clips. Becausestress tends to concentrate at the clips, the film tends to break near aclip first and thus the reported elongation at break will tend to beslightly lower than what may be achieved under a more uniform stressfield. The fully drawn samples tended to break at draw ratios of 1.3×1.3or less, i.e. a biaxial draw ratio of about 1.7. The underdrawn samplestended to strain harden around draw ratios of 1.5×1.5 and tended tobreak around 1.7×1.7, i.e. a biaxial draw fully drawn film ratio of 2.9.A total biaxial draw ratio to break for each film case may beconstructed by multiplying the initial biaxial draw ratio to form thefilm by the biaxial draw ratio to break. The total biaxial draw ratio tobreak for the fully drawn film is thus about 22.4 and for the underdrawnfilm about 25.9. The similarity might be expected given the similarprocess conditions. For example, fully drawn films made at highertemperatures or lower strain rates during the first drawing step, e.g.LO step, often require a higher draw ratio to achieve the same MD indexlevel. Under these altered circumstances, the initial and total biaxialdraw ratios would be higher for the fully drawn film than for theparticular fully drawn film cited in this example. For the fully drawnfilm of this example, the total biaxial draw ratio may be slightly lessthan that of the underdrawn film of this example because the fully drawnfilm was also heat set.

Example 5 Uniaxial Extensibility of a Fully Drawn Film at VariousTemperatures

The extensibility of a fully drawn film made in accordance with Example1 was measured in uniaxial mode for a variety of temperatures using astandard Model #1122 Instron tensile tester available from InstronCorp., Canton Mass. Strips 2.5 cm wide were cut and mounted with aninitial draw gap of 5 cm. Averages were taken over 5 samples and themaximum elongation also noted among the samples. The jaw up speed wasset at 30 cm/second. The results are provided in the following table:

St. Dev. Temperature Average Maximum % Of % Nominal Peak ° C. Elongation% Elongation Elongation Stress (psi) 204 59.4 73.6 10.4 268 177 67.984.6 16.9 386 163 81.0 86.1 5.3 467 149 90.0 116.4 20.9 602 135 82.1110.1 17.8 661 121 89.6 96.2 4.3 888The draw ratio at break is the elongation at break plus unity, i.e. 1.82for 135° C. Notice that the elongation to break is similar at 130° C.and 160° C. as in example 4. The peak stress usually coincided with thebreak stress. This example indicates the utility of elevating thepost-forming temperature to lower the nominal drawing stress, e.g. toobtain greater formability for a given forming stress, e.g. a vacuumpressure. Thus thermoforming at lower pressures to the same extent offinal biaxial draw can be achieved with higher forming temperaturesunder the conditions of this example. This example also indicates areduction in extensibility as the post-forming temperature approachesthe peak crystallization rate temperature. The draw ratio at break isreasonably constant at about 1.85 until the temperature of peakcrystallization is approached (220° C.).

The draw ratios in the preceding table are not the biaxial draw ratiosbecause the width is unconstrained and can neck down during elongation.A purely elastic, incompressible neck down in a true uniaxial draw to1.85 draw ratio would result in a final neck down draw ratio of about0.74 across the sample width and a final biaxial draw ratio of 1.36. Theactual final draw ratio across the sample width was intermediate between1.0 and 0.74, thus the biaxial draw ratio compares favorably with thereported extensibility of the fully drawn film in biaxial mode ofexample 4. Other factors that can effect the comparison include the lessconcentrated stress at the clips which might raise the biaxial drawratio and the uni-directional nature of the extension which might lowerthe biaxial draw ratio.

Example 6 Postforming an Underdrawn Reflective Polarizer Film

A multilayer film of PEN and coPEN was co-extruded, cast and drawn tomake a variety of PEN:coPEN multilayer reflective polarizer films. A0.48 IV PEN (made by 3M Co., St. Paul Minn.) was dried at 135° C. for 24hours and then fed directly into a single screw extruder with exittemperature about 285° C. A 0.54 IV 70/0/30 coPEN (i.e. a copolymer ofPEN formed from 70% naphthalene dicarboxylic acid and 30 dimethylisophthalate proportions by weight, and ethylene glycol; also made by 3MCo., St. Paul) was dried by feeding into a twin screw extruder equippedwith a vacuum and with an exit temperature of about 285° C. Theintrinsic viscosities (IV) were measured on resin pellets using a 60/40weight % phenol/o-dichlorobenzene solvent at 30° C. These resin streamswere co-extruded into a 224 multilayer feedblock set at 285° C. andequipped with an internal protective boundary layer (PBL). Pumping rateswere maintained so that the approximate optical thickness of eachPEN:coPEN layer pair was approximately equal in the optical stack, i.e.an “f-ratio” of 0.5. The PBL was supplied with coPEN in approximatelyone-half the volume as that supplied to the sum of all the PEN layers inthe optical stack. The layer pairs in the optical stack had anapproximately linear gradient in optical thickness. The multilayer stackincluding the PBL was split with an asymmetric multiplier to form twostreams in width ratio of 1.55:1, spread to equivalent widths andre-stacked to form a two packet multilayer stack of 448 layers separatedby an internal protective layer. The multilayer stack including the PBLwas split again with an asymmetric multiplier to form two streams inwidth ratio of 1.25:1, spread to equivalent widths and re-stacked toform a four packet multilayer stack of 896 layers separated by aninternal protective layer. An additional coPEN (IV 0.54) skin was addedto each side of the multilayer stack with each skin layer comprisingabout 10% of the total volumetric flow. The total stream was cast from adie at about 285 C onto a quench wheel set at 65° C. The coPEN skinsrefractive indices were essentially isotropic after casting with indicesof 1.6225 at 632.8 nm as measured by the Metricon Prism Coupler. Thecast thickness was approximately 0.066 cm.

The film was drawn transversely using the laboratory biaxial stretcherof example 2. In each case, the draw ratio in the second in-planedirection was approximately unity. Case 1 was drawn at 130° C. and aninitial rate of 20%/second over 20 seconds to a final measured drawratio of 4.8 in a single draw step. Cases 2 and 3 were made using a veryunderdrawn intermediate. Cases 2 and 3 were drawn to approximately 3.5×,at 130° C., at an initial rate of 20%/second and over a total of 10seconds. These Cases 2 and 3 were then re-heated for 44 seconds at thesecond draw step process temperature, i.e. the post forming steptemperature, and post formed by drawing over 10 seconds in the samedirection as the first step to a final draw ratio of about 4.5. Case 2was re-heated and post formed at 130° C. with a final measured drawratio of 4.6. Case 3 was re-heated and post formed at 175° C. with afinal measured draw ratio of 4.4. Case 4 was made by a similar processto the first drawing step of Cases 2 and 3, i.e. drawn at 130° C. over13 seconds to a final measured draw ratio of 3.8. Case 4 was then heatedfor 65 seconds at 130° C. without re-drawing. Thus Case 4 is indicativeof an underdrawn portion of a final article that undergoes thepost-forming temperatures without additional draw or post-forming heatset. Case 5 was drawn at 130° C. and an initial rate of 20%/second over25 seconds to a final measured draw ratio of 5.4 in a single draw step.Case 6 was made by a similar process to the first drawing step of Cases2 and 3, i.e. drawn at 130° C. over 13 seconds to a final measured drawratio of 3.8. Case 6 was then heated for 65 seconds at 175° C. withoutre-drawing. The following table presents the final index values of thepost formed film as measured using the Metricon Prism Coupler at 632.8nanometers. The draw direction is x, the non-drawn in-plane direction isy, and the thickness direction is z. The calculated total polarizability(TP) is estimated for the birefringent layer, as are the totalpolarizability differences (TPD), the estimated density (in g/cc) andthe fractional crystallinity (X) calculated based on the estimateddensity.

Case n x n y n z TP TPD Density X 1, skin 1.6426 1.6194 1.6110 1, stack1.7067 1.6211 1.5871 1, est. PEN 1.7708 1.6228 1.5632 1.0925 0.00981.3405 0.1437 2, skin 1.6330 1.6228 1.6195 2, stack 1.7053 1.6218 1.59332, est. PEN 1.7776 1.6208 1.5671 1.0969 0.0142 1.3459 0.2139 3, skin1.6254 1.6251 1.6230 3, stack 1.7338 1.6258 1.5720 3, est. PEN 1.84221.6265 1.5210 1.1025 0.0198 1.3528 0.3019 4, skin 1.6315 1.6183 1.61884, stack 1.6859 1.6251 1.5948 4, est. PEN 1.7403 1.6282 1.5710 1.08700.0042 1.3337 0.0564 5, skin 1.6424 1.6187 1.6142 5, stack 1.7251 1.61831.5789 5, est. PEN 1.8078 1.6185 1.5436 1.0966 0.01388 1.3455 0.2088 6,skin 1.6256 1.6225 1.6220 6, stack 1.7254 1.6227 1.5714 6, est. PEN1.8252 1.6229 1.5208 1.0943 0.0115 1.3427 0.1719Case 1 is thus an example of a single step process that makes anunderdrawn film. Cases 2 and 3 begin with an underdrawn intermediary butfinish as fully drawn. Case 4 is approximately that underdrawnintermediary. It represents a low level of effective drawing (e.g.Regime II). Case 5 is a single-step fully drawn reflective polarizer.Case 6 is the underdrawn intermediary re-heated as in a post formingstep without further drawing with a greatly enhanced level of effectivedrawing compared to Case 4 (e.g. Regime III).

The following table summarizes the optical performance of the variousCases:

Ave. Location Blue Red Fractional Minimum of Case Edge Edge TransmissionTransmission Minimum 1 <400 nm 900 nm 0.117 0.003 852 nm 2 413 973 0.1120.012 897 3 403 1012 0.115 0.003 941 4 480 1074 0.199 0.033 992 5 <400885 0.063 0.002 810 6 470 1080 0.109 0.005 840The blue edge is defined as the lower edge of the reflection band wherethe fractional transmission is 0.5. The red edge is defined as the upperedge of the reflection band where the fractional transmission is 0.5.The average transmission is a flat average across the reflection bandfrom the blue edge plus 20 nm to the red edge minus 20 nm. The minimumtransmission is the lowest value measured where the transmissionmeasurement is smoothed over 3 nm, and the location is the wavelength ofthis occurrence. The band positions in part result from the differentbiaxial draw ratios and in part from the varying initial stack thicknessof the cast web. The pass fractional transmissions were uniformly highacross the reflection bands for every case, with band averages ofgreater than 0.86. The difference between this result and unity isaccounted for the most part by surface reflections.

Cases 1, 2 and 3 are all films underdrawn to the final same amount.These cases demonstrate the utility of making an underdrawn film, e.g.Case 4, of low orientation and crystallinity (e.g. total polarizability)which is then subsequently post formed (e.g. into a shaped article).Case 4 underdrawn films can be further post formed as described inexample 7.

Case 6 demonstrates the utility of a post forming heat setting step,e.g. after the shaping of an article by drawing and/or molding. Case 6demonstrates at least the same optical performance as the re-drawnunderdrawn cases. Thus a single article formed from an initiallyunderdrawn film could have both re-drawn and non-drawn areas withsimilar optical performance. This performance compares reasonably with afully drawn film.

FIG. 19 compares the spectra of cases 2, 5 and 6, for the block statesof the reflective polarizer, i.e. the fractional transmission of lightpolarized in the draw direction at normal incidence. A typical passstate, i.e. the fractional transmission of light polarized in thenon-drawn in-plane direction at normal incidence, is also presented.

It should be noted that a homogeneous undrawn cast web of PEN was drawnaccording to the conditions of Cases 1 and 5 at 175° C. The cast filmdrew non-uniformly and remained essentially isotropic. This should becontrasted with Case 3, which was underdrawn to about 3.5 at 130 C andthen re-drawn at 175 C with approximately the same optical effect as theunderdrawn film Case 2 and the single-step underdrawn film Case 1.According to the index measurements, the higher post-forming temperatureof Case 3 could improve the optical performance. Actual performance ofthese cases is also affected by the band widths: wider bands tend to beleakier than narrower bands using the same layer gradient. Dispersion,i.e. the change in index with wavelength, is another factor. The indexdifference between the PEN and coPEN layers in this example tend toincrease with decreasing wavelength. Thus the same stack constructionwill have better optical performance as the red edge shifts to lowerwavelengths.

Example 7 Postforming an Underdrawn Film in Multiple Steps

An underdrawn reflective polarizer film may also be post formed throughmultiple steps. In this example, an undrawn multilayer cast web of PENand coPEN was co-extruded and cast according to example 6. The film wasdrawn transversely using the laboratory biaxial stretcher of example 2.In each case, the draw ratio in the second in-plane direction wasapproximately unity. In case A, the cast web first was drawn at 135° C.and an initial rate of 20%/second over 10 seconds to a measured drawratio of 3.2 in a single draw step. The film of case A could not bepeeled apart using typical methods. The transmission spectra weremeasured using a Perkin-Elmer Lambda-19 spectrophotometer and the samplewas preheated for 25 seconds at 135° C., then further preheated for 25seconds at 160° C. and re-drawn over another 10 seconds to a finalmeasured draw ratio of approximately 4.8. This is case B. A portion ofthe film was destructively peeled and indices measured at 632.8 nm.Transmission spectra were measured using the Perkin-Elmer Lambda-19spectrophotometer. Finally, the sample was again preheated for 25seconds at 135° C., then further preheated for 25 seconds at 160° C. andre-drawn over another 4 seconds to a final measured draw ratio ofapproximately 6.0. This is case C. A portion of the film wasdestructively peeled and indices measured at 632.8 nm. Transmissionspectra were measured using a Perkin-Elmer Lambda-19 spectrophotometer.The following table presents the final index values of the post formedfilm as measured using a Perkin-Elmer Lambda-19 spectrophotometer. Thedraw direction is x, the non-drawn in-plane direction is y, and thethickness direction is z. The calculated total polarizability (TP) isestimated for the birefringent layer, as are the total polarizabilitydifferences(TPD), the density (in g/cc) and the fractional crystallinity(X).

Sample n x n y n z TP TPD Density X B, skin 1.6426 1.6194 1.6152 B,stack 1.7704 1.6185 1.5864 B, est. PEN 1.7704 1.6176 1.5576 1.09080.0081 1.3384 0.1176 C, skin 1.6330 1.6228 1.6195 C, stack 1.7053 1.62181.5933 C, est. PEN 1.7776 1.6208 1.5671 1.0969 0.0142 1.3459 0.2139In these cases, the effect of the second re-drawing step was to increasethe total polarizability and the amount of effective draw with only amodest effect on the index differences.

FIG. 20 presents the block fractional transmissions for the three cases.The strength of the block reflectance band is similar for cases B and C.The band is slightly improved in case C in part due to an increase inthe layer density due to thinning from case B to C.

Example 8 Thermoformed Mirror Film Headlamp

A 35.6 cm.×35.6 cm. (14 inch by 14 inch) sample of polymeric multilayermirror film made according to Example 1 was thermoformed into the shapeof a rectangular headlamp using a Formech 450 Vacuum Forming Machine(obtained from 6 McKay Trading Estate, Kensal Road, London). To start,the controls for heating zones 1, 2, and 3 of the vacuum former were setto level 3, and the apparatus was allowed to equilibrate for at least 30minutes to ensure that the heating plate was at the correct temperature.A room temperature silicone rubber mold in the shape of a rectangularheadlamp (Wagner's Halogen Headlamp H4701 High Beam) was placed in thecenter of the movable platform on the vacuum former, with the longestdimension pointing to the right and left with respect to the operator.The frame of the vacuum former was unlocked and lifted up, and themultilayer mirror film was taped over the open cavity directly above themold and movable platform. The entire perimeter of the film was securelytaped down using 5.08 cm (2 inch) wide Scotch™ brand 471 tape (availablefrom 3M Company, St. Paul, Minn.) to ensure a hermetic seal, which isneeded to maintain vacuum at a later step. It is important to ensurethat there are no wrinkles in the tape that may create channels throughwhich the vacuum might leak. The frame of the vacuum former was thenclosed down and locked to ensure a tight closure.

Two 1.27 cm (½ inch) metal block spacers were placed on the vacuumformer frame's corners closest to the operator in order to allow theheating plate to be raised sufficiently to allow room for the mold. Theheating plate was then slid onto the metal blocks so that the rails ofthe hot plate would lie on the edge of these blocks, and the heatingplate was kept in position for 30 seconds to soften the film. Themovable platform containing the silicone rubber mold was then raised allthe way up so that the mold would deform the multilayer mirror film. Thevacuum was immediately turned on and a vacuum pulled in order to stretchthe film around the mold.

After ten seconds, the heating plate was removed from the sample bylifting a few inches and sliding it back into its original position.Lifting the hot plate is important to avoid burning the film. The filmwas then allowed to cool for about 10 seconds and the vacuum was turnedoff. After about 15 seconds, the movable platform and mold were droppedaway from the film and the metal spacer blocks were removed from thevacuum former. The frame of the vacuum former was then unlocked andlifted to allow removal of the tape and film. This procedure resulted ina thermoformed article with no significant wrinkles or color distortionswhen viewed at a direction normal to the film.

Example 9 Embossed Color Shifting Security Film

A color shifting security film was made and embossed according toExamples 1 and 4 in U.S. Pat. No. 6,045,894 (Jonza et al.), which isherein incorporated by reference. A multilayer film containing about 418layers was made on a sequential flat-film making line via a coextrusionprocess. This multilayer polymer film was made PET and ECDEL™ 9967 wherePET was the outer layer or “skin” layer. A feedblock method (such asthat described by U.S. Pat. No. 3,801,429) was used to generate about209 layers with an approximately linear layer thickness gradient fromlayer to layer.

The PET, with an intrinsic viscosity (IV) of 0.60 dl/g was pumped to thefeedblock at a rate of about 34.0 Kg/hr and the ECDEL™ at about 32.8Kg/hr. After the feedblock, the same PET extruder delivered PET asprotective boundary layers to both sides of the extrudate at about 8Kg/hr total flow. The material stream then passed though an asymmetricdouble multiplier, as described in U.S. Pat. Nos. 5,094,788 and5,094,793, with a multiplier ratio of about 1.40. The multiplier ratiois defined as the average layer thickness of layers produced in themajor conduit divided by the average layer thickness of layers in theminor conduit. Each set of 209 layers has the approximate layerthickness profile created by the feedblock, with overall thickness scalefactors determined by the multiplier and film extrusion rates.

The ECDEL™ melt process equipment was maintained at about 250° C., thePET (optics layers) melt process equipment was maintained at about 265°C., and the multiplier, skin-layer meltstream and die were maintained atabout 274° C. The feedblock used to make the film for this example wasdesigned to give a linear layer thickness distribution with a 1.3:1ratio of thickest to thinnest layers under isothermal conditions. Toachieve a smaller ratio for this example, a thermal profile was appliedto the feedblock. The portion of the feedblock making the thinnestlayers was heated to 285° C., while the portion making the thickestlayers was heated to 268° C. In this manner the thinnest layers are madethicker than with isothermal feedblock operation, and the thickestlayers are made thinner than under isothermal operation. Portionsintermediate were set to follow a linear temperature profile betweenthese two extremes. The overall effect is a narrower layer thicknessdistribution which results in a narrower reflectance spectrum. Somelayer thickness errors are introduced by the multiplier, and account forthe minor differences in the spectral features of each reflectance band.The casting wheel speed was set at 6.5 m/min (21.2 ft/min).

After the multiplier, thick symmetric skin layers were added at about35.0 Kg/hour that was fed from a third extruder. Then the materialstream passed through a film die and onto a water cooled casting wheel.The inlet water temperature on the casting wheel was about 7° C. A highvoltage pinning system was used to pin the extrudate to the castingwheel. The pinning wire was about 0.17 mm thick and a voltage of about5.5 kV was applied. The pinning wire was positioned manually by anoperator about 3-5 mm from the web at the point of contact to thecasting wheel to obtain a smooth appearance to the cast web. The castweb was continuously oriented by conventional sequential length orienter(LO) and tenter equipment. The web was length oriented to a draw ratioof about 2.5 at about 100° C. The film was preheated to about 100° C. inabout 22 seconds in the tenter and drawn in the transverse direction toa draw ratio of about 3.3 at a rate of about 20% per second. The filmwas heat set for about 20 seconds in an oven zone set at 226° C.

The finished film had a final thickness of about 0.08 mm. The band edgeat normal incidence was 720 nm, just beyond the visible edge of 700 nm,so that the film looked clear. At 45 degrees, the band edge had shiftedover to 640 nm, and the film appeared cyan. At 60 degrees, the totallack of transmitted red light made the film a brilliant cyan, due to thehigh reflectance of the multilayer stack even at this angle ofincidence. If this film is viewed where there is only a single lightsource, the specular reflection was evident (red) even with a whitepaper background. When laminated to a black background (no transmittedlight), the red was easily visible. Although this film exhibited thedesired color change, a film of fewer layers and narrower bandwidthwould be more desirable.

The film was then embossed between a roll at 149° C. (300° F.) and apre-heated plate. The film thinned down from 3.4 mils to about 3.0 milsin the embossed regions. A surprising result of this embossing was thehow apparent a gold reflection became. A bright gold was observed in theembossed region changing to cyan or deeper blue as the viewing angle ismade shallower. The appearance was similar to gold leaf, yet (at leastin this example) is not as uniform. Bright red and green were alsoapparent. The dramatic change from gold to blue while the unembossedareas change from clear to cyan provided an overt verification featurethat was more dramatic than a transparent hologram.

Example 10 Vacuum Forming of a Trifurcated Light Guide

A trifurcated light guide was vacuum formed from a highly reflectivePEN/PMMA multilayer mirror that was made as described in Example 2 ofU.S. Pat. No. 6,080,467 (Weber et al.). A coextruded film containing 601layers was made on a sequential flat-film-making line via a coextrusionprocess. Polyethylene Naphthalate (PEN) with an Intrinsic Viscosity of0.57 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered byextruder A at a rate of 114 pounds per hour with 64 pounds per hourgoing to the feedblock and the rest going to skin layers describedbelow. PMMA (CP-82 from ICI of Americas) was delivered by extruder B ata rate of 61 pounds per hour with all of it going to the feedblock. PENwas on the skin layers of the feedblock. The feedblock method was usedto generate 151 layers using the feedblock such as those described inU.S. Pat. No. 3,801,429, after the feedblock two symmetric skin layerswere coextruded using extruder C metering about 30 pounds per hour ofthe same type of PEN delivered by extruder A. This extrudate passedthrough two multipliers producing an extrudate of about 601 layers. U.S.Pat. No. 3,565,985 describes similar coextrusion multipliers. Theextrudate passed through another device that coextruded skin layers at atotal rate of 50 pounds per hour of PEN from extruder A. The web waslength oriented to a draw ratio of about 3.2 with the web temperature atabout 280° F. The film was subsequently preheated to about 310° F. inabout 38 seconds and drawn in the transverse direction to a draw ratioof about 4.5 at a rate of about 11% per second. The film was thenheat-set at 440° F. with no relaxation allowed. The finished filmthickness was about 3 mil. The bandwidth at normal incidence was about350 nm with an average in-band extinction of greater than 99%. Theamount of optical absorption was difficult to measure because of its lowvalue, but was less than 1%.

A 17.8 cm (7 inch) by 25.4 cm (10 inch) by 2.5 cm (1 inch) block of woodwas used to prepare a vacuum forming mold. A series of small holes weredrilled in the lowest part of grooves routed in the wood as shown inFIG. 10. After removing the release liner from one side of an acrylicfoam double sided tape, the adhesive was applied to the periphery on thenon-routed side of the wood block to form a chamber beneath the mold;the second release liner was not removed from the other side of theadhesive tape. The mold was then placed on the vacuum table of a vacuumforming apparatus. The multilayer film was mounted in a heating frame,and the film was heated for 4 minutes beneath an electrical heatingelement to 177° C. (350° C.). The film was then rapidly lowered onto theevacuated mold, drawing the polymer film into the grooved cavity. Thefilm maintained its high reflectivity after the vacuum formingoperation.

While the formed film was still in the mold, double-sided adhesive tapewas applied to the portions of the film that were not drawn into themold. A second sheet of mirror film was then adhered to the formedmirror film. The tips of the four termini were cut off to form an inletwith three outlets as shown in FIG. 10. The terminus of a fiber opticlight fixture was inserted into the inlet of the light guide, and whenlight was directed into the light guide input, light emerged from eachof the outlets.

Example 11 Structured Surfaced Multilayer Optical Film

A coextruded film containing 601 layers of PEN/coPEN was made on asequential flat-film-making line via a coextrusion process as describedin Example 10 of U.S. Pat. No. 5,882,774 (Jonza et al.). A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt %Phenol plus 40 wt % dichlorobenzene) was delivered by on extruder at arate of 75 pounds per hour and the coPEN was delivered by anotherextruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole %2,6naphthalene dicarboxylate methyl ester, 15% dimethyl isophthalate and15% dimethyl terephthalate with ethylene glycol. The feedblock methodwas used to generate 151 layers. The feedblock was designed to produce agradient distribution of layers with a ration of thickness of theoptical layers of 1.22 for the PEN and 1.22 for the coPEN. The PEN skinlayers were coextruded on the outside of the optical stack with a totalthickness of 8% of the coextruded layers. The optical stack wasmultiplied by two sequential multipliers. The nominal multiplicationratio of the multipliers were 1.2 and 1.27, respectively. The film wassubsequently preheated to 310° F. in about 40 seconds and drawn in thetransverse direction to a draw ratio of about 5.0 at a rate of 6% persecond. The finished film thickness was about 2 mils. Samples of thefilm were embossed using four different nickel electroformed tools and alarge hydraulic Wabash Press equipped with a 7.6 cm (3 inch) piston anda platens heated to 191° C. (375° F.).

An X-cut fastener (negative) tool was placed on a 2.54 mm (0.1 inch)thick sheet of aluminum. The mirror film was placed on the tool and thencovered with two sheets of 3 mil polyester terephthalate and anothersheet of 0.1 inch aluminum. The sandwich was placed closed between theheated platens with minimal pressure and the sandwich was heated for 60seconds. A force of 6000 lbs was applied to the sandwich for 60 seconds.After the force was removed, the embossed film was removed from thetool. The post-formed film showed altered colors in the square embossedareas with both transmitted and reflected light due to thinning of themultilayer optical stack.

A linear section of the X-cut fastener tool was placed on a 2.54 mm (0.1inch) thick sheet of aluminum. The mirror film was placed on the tooland then covered with two sheets of 3 mil polyester terephthalate andanother sheet of 0.1 inch aluminum. The sandwich was placed closedbetween the heated platens with minimal pressure and the sandwich washeated for 60 seconds. A force of 6000 lbs was applied to the sandwichfor 60 seconds. After the force was removed, the embossed film wasremoved from the tool. The post-formed film showed altered colors in thelinear embossed areas with both transmitted and reflected light due tothinning of the multilayer optical stack.

An X-cut flat top (positive) tool was placed on a stack of 16 sheets ofnotebook paper because of the rough back of the tool. The tool and paperwere placed on a 2.54 mm (0.1 inch) thick sheet of aluminum. The mirrorfilm was placed on the tool and then covered with two sheets of 3 milpolyester terephthalate and another sheet of 0.1 inch aluminum. Thesandwich was placed closed between the heated platens with minimalpressure and the sandwich was heated for 90 seconds. A force of 6000 lbswas applied to the sandwich for 60 seconds. After the force was removed,the embossed film was removed from the tool. The post-formed film showedaltered colors in the pyramidal embossed areas with both transmitted andreflected light due to thinning of the multilayer optical stack.

A 21 mil cube corner tool was placed on a 2.54 mm (0.1 inch) thick sheetof aluminum. The mirror film was placed on the tool and covered with asheet of ¼ inch silicone rubber. The sandwich was placed closed betweenthe heated platens with minimal pressure and the sandwich was heated for30 seconds. A force of 2000 lbs was applied to the sandwich for 60seconds. After the force was removed, the perforated film was removedfrom the tool. The post-formed film showed altered colors in thehexagonal embossed areas with both transmitted and reflected light dueto thinning of the multilayer optical stack.

The same 21 mil cube corner tool was also used to cold emboss themultilayer optical film. The cube corner tool was adhesively attached toa 0.25 inch sheet of polymethylmethacrylate. The mirror film was placedon the tool and covered with a sheet of ¼ inch silicone rubber. Thesandwich was placed into the press and a force of 2000 lbs was appliedto the sandwich for 10 seconds. After the force was removed, theembossed film was removed from the tool. The post-formed film showedaltered colors in the triangular pyramidal embossed areas with bothtransmitted and reflected light due to thinning of the multilayeroptical stack.

The structured surfaced multilayered films of this example are useful asoptical filters, controlled transmission reflectors, optical diodes,diffuse polarizing/depolarizing reflectors, focussing reflectors,decorative films, and light guides. The thin flexible films can be usedin the same ways as a highly reflective metallized film without worry ofcorrosion and cracking of the metallic thin film upon severe/extremedeformation, embossing, or perforation or the dangers associated withtheir conductivity.

Example 12 Corrugated Ribbons

A post-forming process that may be used to produce a decorative item,such as any of the previously mentioned decorative items, is acorrugation process. FIG. 21 shows an arrangement for corrugating thefilms that includes first and second generally cylindrical corrugatingmembers or rollers 220 and 221 each having an axis and a multiplicity ofspaced ridges 219 defining the periphery of the corrugating members 220and 221. Each corrugating member 220 and 221 is driven by its own drivemechanism. The spaces between ridges 219 are adapted to receive ridges219 of the other corrugating member in meshing relationship with themultilayer optical film 212 inserted therebetween. The arrangement alsoincludes means for rotating at least one of the corrugating members 220or 221 so that when the film 212 is fed between the meshed portions ofthe ridges the film 212 will be generally conformed to the periphery ofthe first corrugating member 220.

Process parameters that influence the decorative appearance of theresulting corrugated films include the temperatures of the corrugatingrollers, the nip pressure between the corrugating rollers, the diameterof the corrugating rollers, the line speed, the shape of ridges 219, andthe number of corrugations per inch that the rollers are designed toproduce. The number of corrugations per inch is determined by thespacing between ridges 219. Specifically, a pair of intermeshing ridgescreates one corrugate. As the examples presented below will illustrate,these parameters may be adjusted to produce different decorativeeffects.

The structure 210 that results from the previously described corrugationprocess is shown in FIG. 22. The undulations may be characterized byarcuate portions 213, valley portions 214, and intermediate portions 215and 216 which connect the arcuate portions to the valley portions. Whilethe undulations shown in FIG. 22 are sinusoidal in shape, it should berecognized that the corrugation process may create undulations of othershapes, such as shown in FIG. 23, for example. In addition, thecorrugates need not extend along the width of the film. Rather, they mayextend in any direction in the plane of the film.

In accordance with one aspect of the present invention, in additions tothe undulations formed by the corrugation process, the corrugationprocess also results in variations in the thickness of the film layers.In particular, the ridges 219 of the corrugating members stretch theintermediate portions 215 and 216 of corrugated film 210 so that theseportions are thinner than arcuate and valley portions 213 and 214.Because of the variations in thickness of the film, the differentportions of the film will reflect light of different wavelengths,producing a noticeable shift in color of the intermediate portionscompared to the arcuate and valley portions 213 and 214. Thisphenomenon, referred to as color or band shifting, occurs because therange of wavelengths reflected by a multilayer optical film is, in part,a function of the physical thickness of the layers in the multilayeroptical film.

Optical Characteristics of Corrugated Films

The pre-corrugated film was fabricated to have a uniform thicknesswithin a specified tolerance (typically about ±5%). When held taut andviewed in normal transmission under fluorescent room lighting, thepre-corrugated film appeared to exhibit primarily a single color, forexample, cyan. Flexure of the film produced substantial changes in thefilm color so that a range of colors were visible along the film. Thatis, the pre-corrugated film exhibited angularly sensitive reflectivecolor filtration. This effect occurs because the film reflects incidentlight in one wavelength range and transmits light in another wavelengthrange, with the wavelength ranges of reflection and transmission varyingwith changes in the angle of incidence of the light. Thus, theparticular color that is observed on a given portion of the film maydiffer from the color observed on another portion of the film becauseflexure of the film causes light to strike the different portions offilm at different angles of incidence. In other words, the number ofcolors that are observed increases as the number of different planesoccupied by various portions of the film increases.

FIG. 24 shows an exemplary pattern observed in normal transmission afterthe film has undergone a corrugation process in accordance with themethod of the present invention to provide the film with an undulatingvariation in thickness. The appearance of the film has changedsubstantially in comparison to the pre-corrugated film. In contrast tothe primarily cyan appearance of the pre-corrugated film (when it istautly arranged without any flexure so that the number of differentplanes which reflect light is minimized), the corrugated film displaysdifferent colored bands that extend in the cross-web direction. Inparticular, bands 320 and 322 of alternating color are formed, withbands 20 appearing in one color (e.g., yellow) and bands 322 appearingin another color (e.g., cyan). Bands 320 correspond to intermediateportions 215 and 216 shown in FIG. 22, which have a reduced layerthickness as a result of the corrugation process, and bands 322correspond to the arcuate and valley portions 213 and 214. In otherwords, the corrugated film has alternating bands or striations ofdifferent colors along its length because of color shifting arising fromthe thickness variations.

When observing light reflected from the corrugated film, the corrugatedfilm appears to have a greater brilliance in comparison to thepre-corrugated film. This is caused by the increased angularity of thefilm produced by the corrugation process. The increased angularityincreases the number of source locations from which light is directedback to the viewer. In addition, the different portions of the filmextend in different planes and light is reflected over a greater rangeof incident angles, which as previously mentioned, results in differentcolors of light being observed.

The corrugating process as employed in the present invention will now befurther described by the following specific examples.

Example 12(a)

A decorative colored mirror film was made using the corrugation processof the present invention. The pre-creped film was prepared from acoextruded film containing 224 layers made on a sequential flat-filmmaking line by a coextrusion process. This multilayer polymer film wasmade from polyethylene naphthalate (PEN) (60 wt. % phenol/40 wt.dichlorobenzene) with an intrinsic viscosity of 0.48dl/g available fromEastman Chemical Company and polymethyl methacrylate (PMMA) availablefrom ICI Acrylics under the designation CP82. PETG 6763 provided theouter or “skin” layers. PETG 6763, believed to be a copolyester based onterephthalate as the dicarboxylate and 1,4-cyclohexane dimethanol andethylene glycol as the diols, is commercially available from EastmanChemicals Co., Rochester, N.Y. A feedblock method (such as thatdescribed by U.S. Pat. No. 3,801,429) was used to generate about 224layers which were coextruded onto a water chilled casting wheel andcontinuously oriented by conventional sequential length orienter (LO)and tenter equipment. PEN was delivered to the feedblock by one extruderat a rate of 24.2 Kg/hr and the PMMA was delivered by another extruderat a rate of 19.3 Kg/hr. These meltstreams were directed to thefeedblock to create the PEN and PMMA optical layers. The feedblockcreated 224 alternating layers of PEN and PMMA with the two outsidelayers of PEN serving as the protective boundary layers (PBLs) throughthe feedblock. The PMMA melt process equipment was maintained at about274° C.; the PEN melt process equipment, feedblock, skin-layer moduleswere maintained at about 274° C.; and the die was maintained at about285° C. A gradient in layer thickness was designed for the feedblock foreach material with the ratio of thickest to thinnest layers being about1.25.

After the feedblock, a third extruder delivered PETG as skin layers(same thickness on both sides of the optical layer stream) at about 25.8Kg/hr. Then the material stream passed through a film die and onto awater cooled casting wheel using an inlet water temperature of about 24°C. A high voltage pinning system was used to pin the extrudate to thecasting wheel at 3.1 meters/min. The pinning wire was about 0.17 mmthick and a voltage of about 4.9 kV was applied. The pinning wire waspositioned manually by an operator about 3-5 mm from the web at thepoint of contact to the casting wheel to obtain a smooth appearance tothe cast web.

The cast web was length oriented with a draw ratio of about 3.1:1 atabout 130° C. In the tenter, the film was preheated before drawing toabout 135° C. in about 30.9 seconds and then drawn in the transversedirection at about 140° C. to a draw ratio of about 4.5:1, at a rate ofabout 20% per second. The finished pre-corrugated film had a finalthickness of about 0.05 mm.

The pre-corrugated multilayer film was fed into the nip between thecorrugating rollers 220 and 221 shown in FIG. 21. The corrugatingmembers had a diameter of about 9.01-9.02 inches, with ridges shaped toform about 7½ corrugations per inch along the length of the resultantcorrugated film. Both corrugating members were heated to 250° F. The nippressure applied between the corrugating members was 50 pounds force perlineal inch (pli), and the line speed was 5 feet per minute (fpm).

The precorrugated multilayer colored mirror film, as observed in normaltransmission under fluorescent room lighting, exhibited randomlydistributed areas of clear, cyan and blue elongated in the crosswebdirection. The resulting corrugated colored mirror film hadsignificantly changed in its visual appearance. As observed in normaltransmission under fluorescent room lighting, both the peak and valleyportions or regions of the corrugated colored mirror film were cyan incolor. The intermediate portions or regions located between the peaksand valleys changed to yellow in color in normal transmission asobserved under fluorescent room lighting. It is believed that thisobserved color change in the connecting regions between the peaks andvalleys was due to film thinning during the corrugation process. Thecaliper of the corrugated colored mirror film in the intermediateregions was measured and found to be thinner than the caliper measuredfor the peak and valley regions. The caliper of the intermediate regionswas also thinner than the caliper of the pre-corrugated multilayermirror film.

The caliper of the pre-corrugated colored mirror film and the caliper ofthe intermediate regions between the peaks and valleys of the corrugatedcolored mirror film were measured in a conventional manner using amanual caliper instrument (Model #293-761, manufactured by MitutoyoCorporation, 31-19, Shiba5-chome, Minato-ku, Tokyo 108, Japan). Thecaliper data was obtained by averaging ten measurements randomly chosenfrom within each film sample. The caliper data for this film ispresented below:

Thickness of precorrugated colored mirror film: 1.54 mils (std dev 0.11)Thickness of intermediate region between the 1.17 mils (std dev 0.33)peaks and valleys of the corrugated film:

Example 12(b)

A decorative colored mirror film was prepared in a manner similar tothat described for Example 12(a) above. The pre-corrugated multilayercolored mirror film 12 was formed from a coextruded film containing 224layers made on a sequential flat-film making line by a coextrusionprocess. This multilayer polymer film was made from polyethylenenaphthalate (PEN)(60 wt. % phenol/40 wt. % dichlorobenzene)) with anintrinsic viscosity of 0.48 dl/g available from the Eastman ChemicalCompany and polymethyl methacrylate (PMMA) available from ICI Acrylicsunder the designation CP82, where PEN provided the outer or “skin”layers. A feedblock method (such as that described by U.S. Pat. No.3,801,429) was used to generate about 224 layers which were coextrudedonto a water chilled casting wheel and continuously oriented byconventional sequential length orienter (LO) and tenter equipment. PENwas delivered to the feedblock by one extruder at a rate of 38.8 Kg/hrand the PMMA was delivered by another extruder at a rate of 30.1 Kg/hr.These meltstreams were directed to the feedblock to create the PEN andPMMA optical layers. The feedblock created 224 alternating layers of PENand PMMA with the two outside layers of PEN serving as the protectiveboundary layers (PBL's) through the feedblock. The PMMA melt processequipment was maintained at about 274° C.; the PEN melt processequipment, feedblock, skin-layer modules were maintained at about 274°C.; and the die was maintained at about 285° C. A gradient in layerthickness was designed for the feedblock for each material with theratio of thickest to thinnest layers being about 1.31.

After the feedblock, a third extruder delivered 0.48 IV PEN as skinlayers (same thickness on both sides of the optical layer stream) atabout 23.9 Kg/hr. Then the material stream passed through a film die andonto a water cooled casting wheel using an inlet water temperature ofabout 29° C. A high voltage pinning system was used to pin the extrudateto the casting wheel at 5.2 meters/min. The pinning wire was about 0.17mm thick and a voltage of about 6.2 kV was applied. The pinning wire waspositioned manually by an operator about 3-5 mm from the web at thepoint of contact to the casting wheel to obtain a smooth appearance tothe cast web.

The cast web was length oriented with a draw ratio of about 3.1:1 atabout 130° C. In the tenter, the film was preheated before drawing toabout 140° C. in about 18 seconds and then drawn in the transversedirection at about 140° C. to a draw ratio of about 4.6:1, at a rate ofabout 15% per second. The finished pre-corrugated film had a finalthickness of about 0.05 mm.

The corrugating members of the corrugating arrangement were shaped toform about 13 corrugations per inch along the length of the corrugatedfilm. Both corrugating members were heated to 250° F., the nip pressurebetween the corrugating rollers was 50 pli, and the line speed was 15fpm.

The pre-corrugated film was cyan in color when observed in normaltransmission under fluorescent room lighting. The resulting corrugatedfilm had changed in visual appearance. As observed in normaltransmission under fluorescent room lighting, the peak and valleyregions and the intermediate regions between the peaks and valleys allremained cyan in color, but the intermediate regions exhibited a deepershade of cyan. Moreover, when observing light reflected from the film,the film appeared much more brilliant than the film described in Example1, giving the film a visual appearance strikingly different from thefilm in Example 1. The increased brilliance presumably occurred becauseof the increased angularity in the film resulting from the formation ofthe peaks and valleys.

Example 12(c)

The corrugated colored mirror film prepared in Example 12(a) was cutinto rolls of film ½ inch in width using a conventional razor blade. A4⅞ inch diameter confetti bow having 31 loops was then formed from theroll of film. The bow was prepared using a Cambarloc bow machineavailable from Cambarloc Engineering, Inc. Lebanon, Mo.

Example 12(d)

The corrugated colored mirror film prepared in Example 12(b) was cutinto ½ inch width rolls, from which confetti bows were prepared, asdescribed in Example 3.

Example 12(e)

A decorative color mirror film was prepared in a manner similar to thatdescribed in Example 12(a). The pre-corrugated multicolored mirror filmwas formed from a coextruded film containing 224 layers made on asequential flat-film making line by a coextrusion process. Thismultilayer polymer film was made from copolyethylene naphthalate (LMPP)comprised of 90 mol % naphthalate and 10 mol % terephathalate as thedicarboxylates and 100% ethylene glycol as the diol with an intrinsicviscosity of 0.48 dl/g and polymethyl methacrylate (PMMA) available fromICI Acrylics under the designation CP71, where LMPP provided the outeror skin layers. A feedblock method (such as that described by U.S. Pat.No. 3,801,429) was used to generate about 224 layers which werecoextruded onto a water chilled casting wheel and continuously orientedby conventional sequential length orienter (LO) and tenter equipment.LMPP was delivered to the feedblock by one extruder at a rate of 46.0Kg/hr and the PMMA was delivered by another extruder at a rate of 35.9Kg/hr. These meltstreams were directed to the feedblock to create theLMPP and PMMA optical layers.

The feedblock created 224 alternating layers of LMPP and PMMA with thetwo outside layers of LMPP serving as the protective boundary layersthrough the feedblock. The PMMA melt process equipment was maintained atabout 265° C.; the PEN melt process equipment, feedblock, skin-layermodules were maintained at about 265° C.; and the die was maintained atabout 285° C. A gradient in layer thickness was designed for thefeedblock for each material with the ratio of thickest to thinnestlayers being about 1:2. An axial rod, as described in filed patentapplication U.S. Ser. No. 09/006,288 (now abandoned), was used to narrowthe bandwidth.

After the feedblock, a third extruder delivered 0.48 IV LMPP as skinlayers (same thickness on both sides of the optical layer stream) atabout 93.2 Kg/hr. Then the material stream passed through a film die andonto a water cooled casting wheel using an inlet water temperature ofabout 18 C. A high voltage pinning system was used to pin the extrudateto the casting wheel at 6.6 meters/min. The pinning wire was about 0.17mm thick and a voltage of about 5.6 kV was applied. The pinning wire waspositioned manually by an operator about 3-5 mm from the web at thepoint of contact to the casting wheel to obtain a smooth appearance tothe cast web.

The cast web was length oriented with a draw ratio of about 3:3:1 atabout 120 C. In the tenter, the film was preheated before drawing toabout 125 C in about 14 seconds and then drawn in the transversedirection at about 125 C to a draw ratio of about 4:3:1, at a rate ofabout 20% per second. The finished pre-corrugated film had a finalthickness of about 0.05 mm.

The pre-corrugated film was cyan in color when observed in normaltransmission under fluorescent room lighting. The resulting corrugatedfilm when observed in normal transmission under fluorescent lightingexhibited a magenta color at the outside edges of the peaks and valleyswhile the remaining regions of the film maintained the cyan color.

Example 13 Point Embossed Colored Mirror Film

A decorative colored mirror film was made by point embossing amultilayer colored mirror film using conventional embossing equipment.The input film used for the embossing was a coextruded film containing224 layers made on a sequential flat-film making line by a coextrusionprocess. This multilayer polymer film was made from polyethylenenaphthalate (PEN) (60 wt. % phenol/40 wt. % dichlorobenzene)) with anintrinsic viscosity of 0.48 dl/g available from the Eastman ChemicalCompany and polymethyl methacrylate (PMMA) available from ICI Acrylicsunder the designation CP82. PETG 6763 provided the outer or “skin”layers. PETG 6763, believed to be a copolyester based on terephthalateas the dicarboxylate and 1,4-cyclohexane dimethanol and ethylene glycolas the diols, is commercially available from Eastman Chemicals Co.,Rochester, N.Y. A feedblock method (such as that described by U.S. Pat.No. 3,801,429) was used to generate about 224 layers which werecoextruded onto a water chilled casting wheel and continuously orientedby conventional sequential length orienter (LO) and tenter equipment.PEN was delivered to the feedblock by one extruder at a rate of 24.2Kg/hr and the PMMA was delivered by another extruder at a rate of 19.3Kg/hr. These meltstreams were directed to the feedblock to create thePEN and PMMA optical layers. The feedblock created 224 alternatinglayers of PEN and PMMA with the two outside layers of PEN serving as theprotective boundary layers (PBL's) through the feedblock. The PMMA meltprocess equipment was maintained at about 274° C.; the PEN melt processequipment, feedblock, skin-layer modules were maintained at about 274°C.; and the die was maintained at about 285°C. A gradient in layerthickness was designed for the feedblock for each material with theratio of thickest to thinnest layers being about 1.25.

After the feedblock, a third extruder delivered PETG as skin layers(same thickness on both sides of the optical layer stream) at about 25.8Kg/hr. Then the material stream passed through a film die and onto awater cooled casting wheel using an inlet water temperature of about 24°Celsius. A high voltage pinning system was used to pin the extrudate tothe casting wheel at 3.1 meters/min. The pinning wire was about 0.17 mmthick and a voltage of about 4.9 kV was applied. The pinning wire waspositioned manually by an operator about 3-5 mm from the web at thepoint of contact to the casting wheel to obtain a smooth appearance tothe cast web.

The cast web was length oriented with a draw ratio of about 3.1:1 atabout 130° C. In the tenter, the film was preheated before drawing toabout 135° C. in about 30.9 seconds and then drawn in the transversedirection at about 140° C. to a draw ratio of about 4.5:1, at a rate ofabout 20% per second. The finished film had a final thickness of about0.05 mm.

The film was passed between two nipped heated embossing rollers. The topembossing roller, which was heated to 250 degrees F., had a raiseddiamond shaped embossing pattern engraved on its surface. The embossingpattern was designed so that 5% of the surface area of the film would beembossed with the diamond pattern.

The bottom laminating roller had a smooth surface and was heated to 250degrees F. The nip pressure was 100 pounds force per lineal inch (pli)and the line speed was 5 feet per minute (fpm).

Prior to embossing, the multilayer colored mirror film exhibitedrandomly distributed areas of clear, cyan, and blue elongated in thecrossweb direction, as observed in normal transmission under fluorescentroom lighting. The resulting embossed colored mirror film had changed inits visual appearance. As observed in normal transmission underfluorescent room lighting, the embossed areas of the film were magentain color, while the film in the areas between the embossed regionsremained similar in appearance to the pre-embossed film, that is,exhibiting randomly distributed areas of clear, cyan and blue elongatedin the crossweb direction. It is believed that this observed colorchange in the embossed areas of the film compared to the non-embossedareas of the film was due to film thinning that occurred as a result ofthe embossing process. Cross sectional scanning electronphotomicrographs (SEMs) taken of the resulting embossed colored mirrorfilm showed that the thickness of the embossed areas of the film wereapproximately 63% of the thickness of the non-embossed areas of thefilm.

The embossed colored mirror film was then slit into ½ inch width rollsusing a conventional razor blade slitting method. A 4.875 inch diameterconfetti bow having 31 loops was then formed from the roll of film. Thebow was prepared using a Cambarloc bow machine (see U.S. Pat. No.3,464,601) available from Cambarloc Engineering, Lebanon, Mo.

The patents, patent applications, patent documents, and publicationscited herein are incorporated by reference in their entirety, as if eachwere individually incorporated by reference. Various modifications andalterations of this invention will become apparent to those skilled inthe art without departing from the scope of this invention, and itshould be understood that this invention is not to be unduly limited tothe illustrative embodiments set forth herein.

What is claimed is:
 1. An article comprising a multilayer optical filmcomprising an optical stack comprising a plurality of layers, the layerscomprising at least one birefringent polymer and at least one differentpolymer, wherein the optical stack comprises a strain-induced index ofrefraction differential along a first in-plane axis and substantiallythe entire optical stack reflects at least about 85% of light of desiredwavelengths that is polarized along the first in-plane axis, and furtherwherein the thickness of the optical stack varies by at least about 10%or more; and wherein the birefringent polymer is a polyester.
 2. Thearticle of claim 1, wherein the thickness of the optical stack varies byat least about 20% or more.
 3. The article of claim 1, whereinsubstantially the entire optical stack reflects at least about 90% oflight of desired wavelengths that is polarized along the first in-planeaxis.
 4. The article of claim 1, wherein the optical stack furthercomprises a strain-induced index of refraction differential along asecond in-plane axis that is perpendicular to the first in-plane axis.5. The article of claim 1, wherein the optical stack further comprises afirst major surface, the first major surface comprising at least onedepressed area formed thereon.
 6. The article of claim 1, furthercomprising a rigid substrate attached to the multilayer optical film.